The correct option is C = 7 + 2n.
From the question given above, we are told that, a notebook costs $7 and divider cost $2 per kg.
So if you buy one notebook, it will cost $7 and
if you buy n packages of dividers, it will cost $2 * n = 2n.
If you add all the costs of the items together, you will have
Cost [C] = 7 + 2n.
