We are going to use the vertical line test to identify the graph that represents a functions. We can see in graphs B,C and D that one x-value has more than one y-value, this cannot be possible for a function. In this case the vertical line test is showing us that in graphs B,C and D there are parts of the graphs where one x-value has more than one y-value.
Therefore, the answer is the option A as all x values have just one y-value.