Answer:
the bottom left graph is a function
Explanation:
for questions like this pertaining a graph, you can use the vertical line test. essentially for a function, an x value cannot have multiple y values. so, the function cannot have any two or more points that lie on the same vertical line.
the top left graph is literally a vertical line, so that doesn't work
the top right is a circle. draw a vertical line and you see it always intersects two points
the bottom right graph draw a vertical line. two points again, right?
only the bottom left works with the vertical line test.