Answer
(a) Is not
(b) Is
Work Step by Step
(a) Is not a graph of a function of x. It fails the vertical line test, for some values of x on the graph have multiple corresponding values of y, which means this can't be a function of x.
(b) Is a graph of x. Unlike (a), there's no place to draw a vertical line such that it intersects the graph more than once.
