Answer
A graph represents a function if each vertical line will pass through the graph in at most one point only.
The given graph does not represent a function because a number of vertical lines (e.g., $x=0$), pass through the graph at two points.
Work Step by Step
RECALL:
A graph represents a function if it every vertical line will pass trough the graph in at most one point only.
Note that vertical lines such as $x=0$ will pass through the in two points.
Thus, the graph does not represent a function.
