Question

Which area of the earth's crust would be expected to be thickest?

under the Atlantic ocean
in the Rocky Mountains
under the Missouri River
inside the Grand Canyon

Answer 1

The thickest part of the earth's crust is under mountains. Whenever there is a volcanic eruption, more material is poured out on the earth's surface from under the crust, so in that sense yes, the earth's crust is increasing slowly.
So under the mountains

Answer 2

The thickest part of the earth’s crust is under mountains. Whenever there is a volcanic eruption, more material is poured out on the earth’s surface from under the crust, so as the years pass the earth’s crust is increasing slowly.

The area of Rocky Mountains is where the earth’s crust would be expected to be thickest.

Answer: In the Rocky Mountains.

I hope this helps, Regards.