D
Relatively prime means that the only common factor of the pairs of numbers is 1
A 17 and 68
factors of 17 are 1, 17
factors of 68 are 1, 2, 4 , 17, ....
Hence 17 and 68 are not relatively prime
B 15 and 231
factors of 15 are 1, 3, 5, 15
factors of 231 are 1, 3, 7, 11, ...
Hence 15 and 231 are not relatively prime
C 21 and 70
factors of 21 are 1, 3, 7, 21
factors of 70 are 1, 2, 5, 7, 10, ..
Hence 21 and 70 are not relatively prime
D 62 and 105
factors of 62 are 1, 2, 31, 62
factors of 105 are 1, 3, 5, 7, 15, 21,
The only common factor is 1, hence 62 and 105 are relatively prime