The answer is:
d
Work/explanation:
In order for a graph to be a function, it has to pass the vertical line test. Here's how it works.
Draw an imaginary vertical line so that it touches the graph. If the vertical line touches the graph only once, then it's a function. However, if the vertical line touches the graph twice or more times, then it's a relation.
#1 is not a function
#2 is not a function
#3 is not a function
#4 is a function
Therefore, the answer is d (the last graph).