Since the equation asks for the total cost of a vase of flowers (y) that goes on the left side. Each flower costs 2.50 and the flower shop depends on the number of flowers ordered, which makes it a variable. Therefore C is wrong because it has 2.50 as a constant.
The ending equation will be y = 2.50x
Written in context this means that the total cost of a vase of flowers is equal to the cost of each flower (2.50) by how many flowers are ordered.
It is shifted 15 units up. Think of how you're solving for y, and by adding 15 to the equation, y increases by 15. If y increases by 15 the graph shifts up by 15. Hope that helps.