Answer:
No
Explanation:
A function requires that one point in the domain corresponds with exactly one value in the codomain. In other words, one x-value should only correspond to one y-value. Seeing the graph, each x-value has two y-values (one on the top half of the circle, another on the bottom), and thus is not a function.
A simple intuitive way to see if a relation is a function is to use the vertical line test: If a vertical line intersects the relation more than once, it's not a function.