Answer: B) Complex numbers
Complex numbers are always in the form a+bi with 'a' and 'b' as real numbers.
If b = 0 and 'a' is nonzero, then a+bi = a+0i = a which is strictly a real number
If a = 0 and b is nonzero, then a+bi = 0+bi = bi indicating that the number is now purely imaginary
