A function is a relation. But a relation is not necessary to be a function. A relation becomes a function if every input just yields an unique output. In this case, we can rewrite the equation as follows.
[tex]y=-x^2+15[/tex]
This equation definitely reflect a relation. In addition, if we plug any x-value in for x, we only get one output for y. So this is also a function.
Thus, the right choice is A.
