Composite numbers are natural numbers greater than 1 that are not prime; they have divisors other than 1 and themselves. Let's evaluate each of the given numbers:
- 11: Prime
- 21: Composite (divisible by 1, 3, 7, and 21)
- 31: Prime
- 41: Prime
- 51: Composite (divisible by 1, 3, 17, and 51)
The composite numbers from your list are 21 and 51.