(a) and (c) are functions
Work Step by Step
The vertical line test determines if a graph is a function. The graphs shown in (a) and (c) pass the vertical line test and therefore are functions. The graphs in (b) and (d) fail the vertical line test. A vertical line would pass these functions in two places, implying that there are two different $y$ values for the same $x$ value, which is not allowed for functions. Thus (b) and (d) are not functions.