Answer: Lower right corner (ie southeast corner)
In this graph, it is impossible to draw a single straight vertical line through more than one point on the yellow line. Therefore, we conclude that this graph passes the vertical line test, which indicates we have a function.
In contrast, the upper left corner fails the vertical line test. Note the left-most pair of points are vertically stacked together. A single vertical line goes through these two points. So this is one possible way to show the graph does not pass the vertical line test, and thereby making this not a function. The upper right corner and lower left corner has the same idea as the upper left corner, so they aren't functions either.