A and C are not arithmetic because they are either dividing or adding. B does not have a pattern so D is the right answer. An arithmetic sequence is a sequence that has a pattern to it. For example, 5, 7, 9, 11, 13, 15 ARE being added by 2.
This is the formula: an = d (n - 1) + c
I hope this helps. It is really easy when you can eliminate answers ;)