The relation that is a function is that represented in the fourth graph. y = 2IxI
You must know that for a relation to be a function, there can not be two different values of f (x) for the same value of x.
In other words, if we have two sets P and M.
A function "f" between them is a relation that assigns to each value of the set P a single value of the set M.
To quickly verify when observing a graph it draws vertical lines and if they cut the graph in two points then the relation is not a function because it assigns two different values of f (x) for the same value of x.
By doing this test you will verify that only the fourth image represents the graph of a function.