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).