Question

sea level
crust
mantle
lava source
item a
item b
how is item a different from item b?
the crust under the ocean in item a is thicker than the crust under the mountain in item b.
the mantle under the ocean in item a is thinner than the mantle under the mountain in item b.
the crust in item a is found partially below sea level as compared to the crust in item b.
the mantle in item a is located far below a mountain as compared to the mantle in item b.

Explanation:

In Item A, part of the crust is below sea - level. In Item B, the crust is entirely above sea - level. Oceanic crust is generally thinner than continental crust under mountains. The mantle's position relative to mountains isn't the main difference here. The key difference is the sea - level position of the crust.

Answer:

The crust in Item A is found partially below sea level as compared to the crust in Item B.