Answer: Upper left corner
Reason:
It is impossible to pass a single vertical line through more than one point on the graph. This relation passes the vertical line test.
The other graphs fail the vertical line test. For instance, in the upper right corner we have a vertical line through x = -1 which passes through more than one point. The input x = -1 leads to multiple outputs y = -1 and y = 3 at the same time.
A function is only possible when each input leads to exactly one output.
