Answer
In (a) and (b) y is a function of x, $\forall x$.
In (c) and (d) y is not a function of x.
Work Step by Step
In (a) and (b) y is a function of x for every x, because there is only one value of y for every x.
In (c) and (d) y is not a function of x, because there are values of x that define more than one value of y.
See 0.1.1 definition.
