If additional weight is added to a piece of crust, how will the crust respond?

The correct answer is - it sinks.

When there's additional weight added to the crust, it starts to sink, the most at the middle section of the pressure, and the least on the sides of the pressure. This thing occurs very often when there's an ice age, as the ice sheets become very large and very heavy, so the crust is sinking. As the ice age ends, and the ice melts, the crust doesn't have the weight on top of it anymore, so it does the opposite thing, it lifts up, or rather, returns to its original shape and height.

the answer would be the third option, "no, ariel should have added 9^2 and 12^2 and compared that to 15^2."

this is found through the pythagorean theorem, which is a^2 + b^2 = c^2.

this is the correct way of solving the problem:

9^2 + 12^2 = 15^2

81 + 144 = 225

225 = 225