Which of the following is a pair of co-primes?

(23, 92)

(21, 35)

(12, 28)

(18, 25)

Co-primes (or relatively prime numbers) are pairs of numbers whose greatest common divisor (GCD) is 1.

Let's check the GCD for each pair:

(23, 92): GCD = 23 (not co-prime).

(21, 35): GCD = 7 (not co-prime).

(12, 28): GCD = 4 (not co-prime).

(18, 25): GCD = 1 (they are co-prime).