A function has only one y-value for each x-value. If you find a set that has an x-value used more than once, it is not a function, because that x-value is now paired with more than one y-value.
The fourth answer is the only one that has the same x-value used more than once.
Side note: Don’t worry if a y-value is used more than once. That’s totally fine. It’s all about the x-values.
