From the following group of letters, find the group of letters which is different from others:

Logic: In options A, B, and C, the 1st two letters are of opposite case, and the last two letters are also of opposite case.

Explanation:

a) yB → lower–upper, Dw → upper–lower
b) sH → lower–upper, Jq → upper–lower
c) vE → lower–upper, Gt → upper–lower
d) iPRO:
iP → lower–upper, but RO → upper–upper (not opposite)
Thus, the odd one out is (D) iPRO.