Only the last picture represents a function. In fact, a function can associate only one output for every given input.
Graphically, this means that every vertical line must cross the graph only once.
The first graph is literally a vertical line, so it can't be a function.
The second graph can't also be a function, because every vertical line drawn for [tex]x>0[/tex] will cross the graph in two points.
The last graph is a function, because every vertical line will cross it only once.
