Which pair of number is relative prime
7 and 42
16 and 39
66 and 77
90 and 891

The relative prime pair is 16 and 39.

This is because between the two numbers their greatest common factor is 1.

Factors of 16 are 1, 2, 4, 8, 16
Factors of 39 are 1, 3, 13, 39,

The rest of the pairs have more common factors than 1.

relative prime are also called coprime or strangers.