Answer:
bottom right (looks like a vertical parabola opening up)
Explanation:
Use the vertical line test.
Think of a vertical line moving from left to right over the graph.
If at any position of the vertical line, the vertical line intersects more than one point of the graph, the graph fails the vertical line test, and the graph is not a function.
The only graph that passes the vertical line test is the graph at bottom right.
Answer: bottom right (looks like a vertical parabola opening up)