Answer:
Top left
Step-by-step explanation:
A function describe a relation between elements of a set (domain) to one and only one element of a second set (range). The key word is the "one and only one: it means that every time I put a value into the function (imagine it as a box) it returns either no value, or one value.
That lead us to the "vertical line test" or "ruler test". If you scan the images with a vertical line (you can use the edge of a piece of paper, a pen, whatever you want) and slide it across the plane, the moment you touch the graph in two (or more!) points, you can rule out that it's not a function. That's what happens with all graphs besides top left.
