Concept:Two numbers are co-primes if their highest common factor (HCF) is 1.Explanation:Check each pair:A. (16,62): HCF is 2, so not co-prime.B. (23,92): HCF is 23, so not co-prime.C. (18,25): HCF is 1, so they are co-prime.D. (21,35): HCF is 7, so not co-prime.Answer:(18,25) i.e. option C.