Four ranks have been given, out of which three are alike in some manner and one is different. Select the odd one.

Let's analyzed all the options:

(a) General → A high-ranking officer in the Army.
(b) Colonel → A senior officer in the Army.
(c) Brigadier → A rank in the Army, above Colonel and below Major General.
(d) Admiral → A high-ranking officer in the Navy.
Odd One:
General, Colonel, and Brigadier belong to the Army. 
Admiral belongs to the Navy, making it the odd one out

Thus, the correct option is (d) Admiral