Answer: The bottom option, the graph.
Explanation:
A relation is something that gets input from a set, called domain (usually the x) and "transforms" them into an output, from a set called the range.
Now, a relation is only a function if, for all the elements x in the domain, each one of them has only one element y in the range associated to it.
For example, in the first set we can see that the value x = 1 appears two times, it is related to different values in the range, then this is not a function.
The second relation is not a function.
the third relation is:
x = 3*y^2 - 7
This is a function, but in this relation x is the output and y the input, so this is a function of y.
In the fourth relation we can see a graph, this is a function (each value in the domain is related to only one in the range), and in the horizontal axis we can see a x, so this is a function of x.