Answer:
Graph 3 represents a function
Graph 4 is not a function
Explanation:
By definition :
A function is a rule or law which relates all the elements of one set with some UNIQUE element of another set.
Which means that from a given value of the input x, there will be only one value of y in a function f(x) = y
Graphically it would represent that for a given point x , there would only be one value of y corresponding to that x.
In graph 4, we can see that for one x, there exists two values of y.
Example : at x = 5, we have y=5 AND y= -5; which is not possible in case of a function.