1st graph
A simple way to test if a relation may be taken as a function is by applying the Vertical Line Test. If a Vertical Line crosses the graph only once, then it is a function. In this question, only the first one can be considered to be a function.
Because in other words, only the first graph shows one value for x corresponding to another for y value. Not the case for the second and the third graph displaying two values for x for each y value.
