A function means that we get one input and one output. In this example that means for every x-value that we have, we only have one y-value.
Looking at the first option, we can see that there is no x-value that has more than one y-value which means that it is a function.
The rest of the options are seen with at least two values for one x-value in one of the points. Therefore, they wouldn't be considered a function leaving us with only option A as the solution.
