A function will not have repeating x values, and the graph on the bottom right is the only that passes the vertical line test(meaning there are no repeating x values).
Remember a function is a line that only gets intersected once, in this case it's not option one because the graph gets intersected twice. It's not option two because the lines also get intersected twice, it's not option three because it doesn't intersect the line at all which gives you the fourth option which intersects the line once making it a function.
