Line b is the answer, hope this helped
By induction:
It's true for

, since

clearly contains a factor of 3.
Suppose it's true for

, that

is divisible by 3. Then

where

is an integer. This reduces to

and both terms are clearly multiples of 3. We know that

is an integer since we had set

previously, which implies

is a multiple of

. So the statement is true.
You might want to see what other people say but I think that it is 40
320 is the correct answer