Four options are given below. Three of these options are similar to each other because they follow the same pattern. Identify the option that is dissimilar to the other three ?

The general sequence is :
Position of the first letter in the alphabet sequence + Position of the second letter in the alphabet sequence = the number at the end of each term.

Thus, AE6 => 1 + 5 = 6
FD10 => 6 + 4 = 10
DE9 => 4 + 5 = 9
Going by this logic, AD6 does not fit the sequence. It should have been AD5 (1 + 4 = 5).
Therefore, the option that is dissimilar to the other three is AD6.