You need to finish an exhibit. Write your proposal. Explain the overall thesis or theme of your exhibit, and pick five different objects from between 1920 and 2018. Write a description for each. How will you display them? What story will these five objects tell about American life over the last hundred years, and how will your exhibit help people better understand their part in that story?

From what I can see, you are asking about crafting an exhibit proposal.
Ideally, such a proposal should begin with an artist's statement. Here is an example of what an artist's statement should include. Be sure to describe the philosophical underpinnings of your chosen sculptures and also what materials were used to create the objects.
In the body of your proposal, include statements on how many works you will highlight (5), what your sculptures say about American life over the last hundred years, and how your sculptures will help your audience better comprehend their part in the national narrative.
Of course, it is a good idea to decide which sculptures to showcase before you begin your proposal. Let us pick some important American sculptors. Alternatively, you may also decide to include sculptors who are mentioned in your textbook.
For now, we will look at Daniel Chester French (1850–1931), Gutzon Borglum (1867–1941), Georgia O'Keefe (1887–1986), Andy Warhol (1928-1987), and Carole Feuerman (born in 1945).
Before you describe the sculptures, it is a good idea to explain why you picked the works of certain artists. For example, I have chosen works from the beginning of the period you mention (1920), the middle of the period (1940–1970), and the end of the period (2000–2018). Such a range catalogs the progression of American life in the last 100 years and the corresponding changes in public perspectives and preferences. The works particularly highlight how Americans share common interests and how those interests unify them. They also reveal how national perspectives evolve without affecting the underlying ideals that Americans hold dear.
Daniel Chester French: French's most famous sculpture was the Lincoln Memorial, which was dedicated in 1922. The statue highlights Lincoln's strength of character and is an eternal reminder to Americans of his place in the American narrative. Be sure to read about Lincoln's accomplishments and discuss them here. Next, read about the materials used for the Lincoln Memorial: granite, limestone, and marble. The National Park Service also discusses what different parts of the sculpture represent.
Gutzon Borglum: Borglum's most famous work is Mount Rushmore. Be sure to read about the 400-foot high and 500-foot wide granite mountain that now has the faces of four presidents carved into it. The presidents are Theodore Roosevelt, Thomas Jefferson, George Washington, and Abraham Lincoln. The presidents represent important reference points in American history. For example, George Washington led the fight for independence and was the first president of the United States. Meanwhile, Roosevelt fought for the common man by abolishing corporate monopolies (the Square Deal). He also practiced what he called "big stick diplomacy": "Speak softly, and carry a big stick." Such diplomacy involves a combination of calm negotiation and the implicit threat of military force to back up one's position. Of course, you cannot showcase Mount Rushmore in an art gallery, but perhaps a photograph will suffice.
Georgia O'Keefe (1887-1986) or Alexander Calder (1898-1976): You can include either of these two artists. One of O'Keefe's most famous sculptures was Abstraction (1946). O'Keefe was considered one of the pioneers of modern feminist art. Meanwhile, Calder was known for kinetic art, a type of modern, abstract art. Both artists represent the modern American fascination with time and space and how science defines our perspectives. By the way, Calder was also known as one of the pioneers of kinetic art. Here is an interesting article about decoding Calder.
Andy Warhol: Warhol was the leading pioneer in the American Pop Art Movement. He focused on the American fascination with popular culture and the effect mass media consumption has on the American psyche. There are many popular examples of his art; you are probably familiar with a few of his sculptures: the Campbell's soup cans (1962) and the Heinz Tomato Catsup Box (1964). Warhol's sculptures exposed rising commercialism and the corresponding increase in materialism in American society.
Carole Feuerman: Feuerman is known for her hyperrealistic sculptures. Hyperrealistic art depicts what is called simulated reality, a reality that is more stark than what we see with the human eye. This kind of art is all about possibilities, which is certainly a very American philosophy about life. This type of art also draws the audience's attention to recent, leading-edge technological inventions in fields such as medicine and AI.
The above sculptures show that America has always been about possibilities. From the sculptures, our audience may find both comfort and inspiration to enact meaningful changes in their lives.
http://www.visual-arts-cork.com/sculpture/american-sculptors.htm