Which of these is NOT equivalent to 2^-3

A. -2^3
B. 2(1/4)^2
C. 2^2 x 2^-5
D. (1/2)^3

The question is asking which of the answer choices is NOT equivalent to
  2^-3 = 1/(2^3) = 1/8

You can determine this by expressing each of the answer choices in a manner that lets you compare them to 2^-3 or to 1/8.

A. -2^3 = -8 . . . . . this is not equivalent to 2^-3 or 1/8. This is the appropriate selection.

_____
Let's check the others, just to be sure.
B. 2(1/4)^2 = 2(1/16) = 2/16 = 1/8 . . . . is equivalent to 1/8

C. 2^2 × 2^-5 = 2^(2-5) = 2^-3 . . . . is equivalent to 2^-3

D. (1/2)^3 = 1/2^3 = 1/8 . . . . is equivalent to 1/8

Our choice of A is correct, as all these other choices are equivalent to the given expression.