Answer: Graph C
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Reason:
This graph passes the vertical line test, so it is a function.
The vertical line test is where we try to pass a single vertical line through more than one point on the curve. If such a thing is possible, then we say the curve fails the vertical line test and it's not a function.
Graph D for instance can have a vertical line through x = 1 and intersect the curve at (1,4) and (1,-4) simultaneously. The input x = 1 leads to multiple outputs (y = 4 and y = -4). Graph D fails the vertical line test for this very reason, and it is not a function. Graphs A and B are similar stories. Any vertical line itself is automatically not a function.
To have a function, any x input in the domain must lead to exactly one and only one y output in the range.
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Notice with graph C it is impossible to have a single vertical line intersect more than one point on the curve. So this is why graph C passes the vertical line test. Each input leads to exactly one output.