Yes, 15 is a rational number. By definition, a rational number is any number that can be expressed as the quotient or fraction of two integers, where the denominator is not zero.
In this case, 15 can be represented as 15/1. Here, both 15 (the numerator) and 1 (the denominator) are integers, and since the denominator is not zero, this qualifies 15 as a rational number.
Rational numbers include whole numbers, fractions, and repeating or terminating decimals. As such, since 15 can fit into these categories, it is indeed a rational number.