Which of the following is not a perfect square?

A. 36

B. 45

C. 64

D. 81 ... ?

In your question, 45 is not a perfect square.

A perfect square is when you divide or multiply the number by itself.

36 is a perfect square because it's factor is 6 x 6 (6,6).

64 is a perfect square because it's factor is 8 x 8 (8,8).

81 is a perfect square because it's factor is 9 x 9 (9,9).