A composite number is a positive integer that has at least one positive divisor other than one and itself, meaning it can be divided by numbers other than 1 and itself.
From the numbers provided:
- 71 is a prime number (only divisible by 1 and 71).
- 51 is a composite number (divisible by 1, 3, 17, and 51).
- 31 is a prime number (only divisible by 1 and 31).
- 61 is a prime number (only divisible by 1 and 61).
The composite number in the list is 51.