Answer:
A.
Why:
Use the vertical line test. Essentially, if there are any points with the same x-coordinates but with different x-coordinates, it's not a function. If that doesn't make sense, if there is a line or point directly above or below another line or point, it's not a function. For example, B. has two lines underneath each other, with the same x-coordinates but different y. That means it's not a function.