I believe D is the answer because 10^-3 = 0.001 ; there is only one answer that has 0.001 in the correct exponential form (10^-3).
Also, for the future, to correctly figure out what the exponential form, try to take the number given, let's say 10^-2 (^ means to the power of), and move the decimal place to the left, two decimal places (because of the power of -2).
