Okay, here we have this:
Considering that in a function the elements of the domain set never repeat themselves. In other words, each value of x corresponds to a single value from the set y.
We can see in the options that the only relation that satisfies this condition is option A, in the others, at least one value of x corresponds to more than one value in the set y.
So, finally we obtain that the correct answer is the option A.