Which pair of numbers is relatively prime?

Relatively prime numbers have no common factor greater than 1. The factors of 4 are 1, 2, and 4, while the factors of 15 are 1, 3, 5, and 15; their only common factor is 1. The other pairs share a factor greater than 1, so the pair 4 and 15 is relatively prime.

AI-written and checked against the verified answer.