What exhibit does Holden visit at the American Museum of Natural History? He visits the dioramas.
In The Catcher in the Rye, Holden recalls visiting the museum during childhood and thinking about its familiar displays. He is especially interested in the habitat dioramas, which present carefully arranged animals and natural settings behind glass.
The museum matters to Holden because its exhibits never seem to change. The scenes remain frozen and orderly, unlike the unpredictable human world he finds confusing and painful. His memory of the dioramas therefore carries both comfort and unease.
The planetarium, dinosaurs, and mummies are associated with the museum generally, but they are not the exhibit Holden describes visiting in the novel. The canonical answer is dioramas.