Which of the following graphs represents a function? graph a (parabola) graph b (hyperbola) graph c (circle) graph d (horizontal parabola)

Graph A is a function. A function is defined as a graph that has one x for every y value. To verify this, you can use the vertical line test. When a vertical line is held to any portion of the graph, it should intersect the graph only once. This is only true for a parabola, proving it is the function.