B is the odd clock.
Looking at the pattern it is evident that we are switching (or mirroring) the digits representing the hour, and those representing the minute independently.
A is 12:11 the numbers representing the hour become 21 and those for the minutes 11 (since we have the same digits) so we get its counter clock which is E) 21:11
Same for C which is 11:15 after mirroring it becomes 11:51 which is clock D
And we are left with B) 11:21 which is the odd clock since an equivalent one would be 11:12 which is not part of the 5 clocks.
RE: Maths Brain Teasers 64:: Can You - Eliminate the Odd Clock Out???