Answer
a.
Work Step by Step
A function is defined as something that cannot have two different outputs for the same input; in simpler terms, that means that there cannot be a singular x value where there are two or more points. A way to figure out if something is a function or not is the vertical line test. We check to see if there is any overlap, and if there is overlap, it is not a function. We can do this test and find that a. is the only function.
