This problem is solved by this theorem:

If n is divisible by r, and r is divisible by s, then n is divisible by s.

The correct answer cannot be A, B, or C because of the above theorem
since 8 is divisible by 2, 4 and 1.  (Note that n means n/1) 

So that only leaves D.  But let's see why n/6 CANNOT be an integer.

If n/6 were an integer, then n would have to be divisible by 3, 
by the above theorem since 6 is divisible by 3.  But this 
contradicis the given statement that n is not divisible by 3. Thus
the assumption that n/6 is an integer is false.

Edwin