(21 - 49) , (24 - 64) , (25 - 54) , (81 - 36)
Option:
Justification: This is the only pair in which the two numbers have no common factor.

(21 - 49) , (24 - 64) , (25 - 54) , (81 - 36)
Justification: This is the only pair in which the two numbers have no common factor.