Not sure if I just don't understand the question, because I find more than one answer possible.
The question is:
Find the nth term of the arithmetic sequence: 7, 12, 17, 22,...
With (a), I get n=1, 2, 3, 4 which seems to work.
With (b), I get n=0, 1, 2, 3 which also seems to work.
With (c), I get n=.5, 1, 1.5, 2.
With (d), I get n=0, .5, 1, 1.5.