Out of the four number-pairs listed, three are alike in some manner and one is different. Select the odd one.

Logic: In three pairs, 1st number is even, and 2nd number is odd. In one pair, 1st number is odd and 2nd number is even.​

Let's analyze all the options:

Option (a) (6, 35) → Even, Odd
Option (b) (4, 23) → Even, Odd 
Option (c) (15, 30) → Odd, Even (Different)
Option (d) (18, 85) → Even, Odd
Thus, the correct option is (c) (15, 30)