An area where the earth's crust would be expected to be thin would be

under the pacific ocean
in the rocky mountains
under the mississippi river
in the plains region of the midwestern u. s.

I believe the correct answer would be the first choice: under the Pacific Ocean. You have to think that the Pacific Ocean is very, very deep, and it's bottom is deeper than the Rocky Mountain, the Mississippi River, and the plains. Therefore, the crust won't be as thick under the Pacific Ocean.