A graph can be said to be a function when no vertical line intersects the graph in more than one point.
If any vertical line drawn would intersect the line/curve more than once, then the graph does not represent a function. This is called the vertical line test.
From the graphs given, the grap that would not pass the vertical line test is the graph in option Y.
Therefore, the graph in option Y does not represent a function.
You can see the blue vertical on graph Y line intersects the line of the graph on two points, therefore we can say this graph fails the vertical line test.
ANSWER:
Y