(6n^2 + 6n) = 6n(n + 1), which is divisible by both 6 and 12, since n(n+1) is always even.
If n is a natural number, then (6n^2 + 6n) is always divisible by

[1] 6 only
[2] 6 and 12 both
[3] 12 only
[4] 18 only
[5] 12 and 18 both