A. Not a function since x = 8 repeats. We have x = 8 pair up with y = 3 and y = 7 at the same time.
B. Not a function for similar reasons. Now x = 2 repeats. We have x = 2 pair up with y = -3 and y = 7
C. Not a function. We have x = -3 pair up with both y = 6 and y = 9 at the same time.
D. This is a function. Each x value is listed once and only once. Any input (x) leads to exactly one output (y). For choices A through C, it is possible to draw a vertical line through more than one point, thereby failing the vertical line test. In choice D, it is not possible to draw a vertical line through more than one point of this list. So this relation passes the vertical line test.
Answer: Choice D