For an real number k, which of the following must be a real number?
If it’s possible to make the denominator equal zero, then you can eliminate the choice. (A) is not real when k = 0. (B) is not real when k = -1. (C) is not real when k = 1. (E) is not real when k = 1 or -1.
(D) is the answer because there’s no value of k that makes equal zero.