Question

Which of the following is a complex number?

Answer 1

You'll be disappointed to find out these are *all* complex numbers, because the set of real numbers is a subset of the complexes.

Probably the question means to ask, "which of these numbers is strictly non-real?" in which case D would be the answer because it contains the square root of a negative number that introduces the imaginary unit i. All the other options are strictly real numbers.