B is not a real number.
Real number are numbers that are rational (1, 2, 3, -7, -10 etc) and irriational (pi, 9.7582, 1/7, etc), and can be virtually any number, aside from imaginary numbers. Square roots consist of the same number squared, for example the square root of 9, is 3 or the square root of 10 is 3.162... Both of which are real numbers.
A.) The square root of 1, is 1. 1 is a real number.
B.) The square root of -4 is imaginary. There is no real number that can be squared to get -4.
C.) The square root of 16 is 4. 4 is a real number.
D.) The square root of 13 is 3.605... 3.605... is a real number.
Thus meaning your answer is B.
I hope this helps!