Total magnification when the objective is at high power (also called 40x) is:

A 40x

B 100x

C 400x

To find the total magnification, you multiply the power times the power of the ocular lens. The ocular lens is usually 10x power. So, if the power is 40x, you would multiply 40x*10x. That would make the total magnification 400x. So, your answer is c.