Which number is prime? 4. 13. 26. 15.

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, etc.

so 13

13. A prime number is a number which cannot be factored anymore. 4 can be factored as 2 x 2, 26 can be factored as 13 x 2, and 15 can be factored as 3 x 5.