Answer:
the bottom left one (the graph that looks like a circle) is not a function, but the other ones are.
Explanation:
A function is defined so that for every x value within a function has a y value. This means that an x value CANNOT have more than one y value. The bottom right one has two y values for almost every x value within the domain. The top ones are functions, because all x values have only 1 y value (a discontinuous function is still a function!) And the bottom right is too.