Question

an area where the earths crust would be expected to be thin would be
under the pacific ocean
under the mississippi river
in the plains region of the midwestern u.s.
in the rocky mountains

Explanation:

Oceanic crust is generally thinner than continental crust. The Pacific Ocean has oceanic crust. The Mississippi River area, the plains region of the Midwestern U.S., and the Rocky Mountains are on continental crust which is thicker.

Answer:

under the Pacific Ocean