Select the option that is different from the rest.

Logic:  The square of the 1st digit is equal to last two digits.

Option (a): 981 = 9² = 81.

Option (b): 749 = 7²= 49.

Option (c): 636 = 6² = 36.

Option (d): 868 = 8² = 64$$\neq$$ 68.

Thus, the correct option is (d) 868.