Which of the following is not a real number

a. square root - 4

b. square root 16

c. square root 1

d. square root 13

A. square root - 4

Step-by-step explanation:

The answer is A because the square root of - 4 is 2i which is an imaginary number.

-The square root of - 4 is 2i (an imaginary number)

-The square root of 16 is 4

-The square root of 1 is 1

-The square root of 13 is ≈ 3.606