A figure that correctly demonstrates using a straight line to determine that the graphed equation is not a function of x is: C. graph C.
In Mathematics, a vertical line test is a technique which is used to determine whether or not a given relation is a function.
According to the vertical line test, a vertical line must cut through the x-coordinate (x-axis) on the graph of a function at only one (1) point, in order for it to represent a function. Else, the relation does not represent a function because it can only have one output value (y) for a unique input value (x).
In this context, we can reasonably and logically deduce that the relation in graphed C correctly demonstrates an equation that is not a function because the vertical line passes through both point (1, 4) and point (1, -4).
In conlusion, the vertical line test in graph C demonstrates that the input value (1) is mapped to multiple output values (-4, 4).