1. The problem asks to identify which number among the options is a prime number. 2. Recall that a prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. 3. Check each option: - a. 1 is not prime because it has only one positive divisor (1). - b. 13 is prime because its only divisors are 1 and 13. - c. 9 is not prime because it is divisible by 1, 3, and 9. - d. 21 is not prime because it is divisible by 1, 3, 7, and 21. 4. Therefore, the prime number among the options is 13. Final answer: 13