Which of the following pairs of numbers is co-prime?

A co-prime pair has HCF 1. Here 25 and 55 share the factor 5, 34 and 51 share 17, and 39 and 65 share 13. But $27=3^3$ and $64=2^6$ have no prime in common, so HCF(27, 64) = 1. Note that both numbers are composite - being co-prime does not require either number to be prime.