Which of the following is a rational number?

The correct answer is √81.

Explanation: A rational number is defined as a number that can be expressed as the quotient of two integers (a fraction). The square root of 81 is 9, which is an integer. Therefore, √81 is a rational number. In contrast, √45, √77, and √99 are not integers and cannot be expressed as simple fractions, making them irrational numbers.