Answer:
B
Explanation:
this is just common sense. if you would have tried you could have easily answered this yourself.
the sequence starts with 6. that is a1.
so, the formula simply has to add "something" to 6.
therefore, A and D are out.
and then we see immediately that the sequence is built by subtracting 4 (or adding -4) for every step.
so, we need to add multiples of -4.
therefore, only B is correct (C adds multiples of +4).