The independent variable (in this case) is the increase in the price for tickets and goes along the x axis. How the sales respond goes along the y axis. That's why it is called the dependent variable. You do something, whatever you are measuring responds. The response depends on what you do.
So the answer is D. But be careful. There is a snake in the grass in A. It is almost right, but the problem says that they are tracking the ticket sales (in dollars) with respect to increases in the price of each ticket.
There might be fewer people, but they are willing to pay more.
A is not quite correct given the criteria of the problem. The criteria is the selling price of the ticket after the price increases.