Four number-pairs have been given, out of which three are alike in some manner and one is different. Select the number-pair that is different.

21 : 154

The logic followed here is :

Ist number × 7 - 3 = IInd number

31 : 214 → 31 × 7 - 3 = 214

37 : 256 → 37 × 7 - 3 = 256

21 : 154 → 21 × 7 - 3 = 144 ≠ 154 (odd)

39 : 270 → 39 × 7 - 3 = 270