Answer:
the third (bottom left) option
Explanation:
for every valid value of x there must be exactly one value of y.
the first option is valid only for positive x, but then for every valid x there are always 2 possible y values (a positive and a negative one).
the second option is defined for only one value of x (3), but this x has infinitely many y values.
the fourth option is a circle defined for -r <= x <= r (r being the radius). in this defined interval almost every x has 2 assigned y values (only x = -r and x = r have one y value).