All of them sound OK.
All of them are grammatically OK.
There is a slight difference between them, though. It pertains to how the time frame of the events described relate to the present time. Which one to use depends on the exact context (specifically, tense) of the surrounding sentences.
For example, (a) describes a past event (the classmates not knowing) of a matter which is applicable to the present time (as of the present time, Ken's decision has been made). In contrast, (c) describes a past event (the classmates not knowing) of a matter which was applicable to some point in time in the past, and may or may not be applicable now, in the present time.