Yes, 9 is a rational number. A rational number is defined as any number that can be expressed as the quotient or fraction of two integers, where the denominator is not zero.
In the case of 9, it can be written as
9/1, which clearly shows that it is the ratio of two integers: 9 (the numerator) and 1 (the denominator). Since both numbers are integers, and the denominator is not zero, it fits the definition of a rational number.
Rational numbers include whole numbers, fractions, and integers. Therefore, 9, being a whole number, is also a rational number.