C. 1.15<em>x</em> because adding 15% to the cost of the haircut is the same as multiplying the cost by 1.15.
Step-by-step explanation:
This answer is correct because, adding 0.15x (or 15%) to the total amount is the same as multiplying the cost by 1.15. This is because, the 1 that is added to 0.15 represents the total, since we are just multiplying instead of adding to it.