Answer: Graph A (horizontal line in upper left corner)
This graph is a function because it is impossible to pass a single vertical line through more than one point on this graph. Contrast this with something like graph B, and it is possible to draw a vertical line through more than one point. The same happens with C as well. Graph D is a vertical line, and any vertical line is never a function.
Algebraically, for any input x, there is only one output y assuming x is in the domain. For a graph like choice C, plugging in x = 0 leads to y = -3 and y = 3 at the same time, which shows it is not a function.
