Explanation

A composite number is a positive integer that has at least one positive divisor other than 1 and itself.

Let's examine the numbers:

4: divisors are 1, 2, and 4 (composite)

9: divisors are 1, 3, and 9 (composite)

15: divisors are 1, 3, 5, and 15 (composite)

Since all numbers (4, 9, 15) have divisors other than 1 and themselves, they are composite numbers.

The group 4, 9, 15 consists entirely of composite numbers.