Discussion

Explanation:

Co-prime numbers are those pair of numbers which have no common factor apart from 1.

Option (a): 12 and 99 have 3 as common factor, hence they are not co-prime

Option (b): 36 and 81 have 9 as common factor, hence they are not co-prime

Option (c): 25 and 36 have no common factor apart from 1, hence they are co-prime

Hence, option (c).

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