Which of the following is the square root of 81?

The square root of a number is a value that, when multiplied by itself, gives that original number. In this case, we are looking for the number that, when multiplied by itself, results in 81.

If we take the number 9 and multiply it by itself (9 × 9), we get 81. Therefore, 9 is indeed the square root of 81.

The other numbers can be assessed for clarity: 7 multiplied by itself is 49, 8 multiplied by itself is 64, and 10 multiplied by itself is 100. None of these results equate to 81. Hence, 9 is the only number that meets the requirements of being the square root of 81.