Answer:
Option D.
Explanation:
A relation is called a function if there exist a unique output for each input.
Vertical line test : The graph of a function intersect each vertical line almost once.
If the graph passes vertical line test, then it is a function otherwise it is not a function.
From the given graphs it is clear that graph first three graphs intersect the vertical line more than once, therefore they are not functions.
Last graph passes the vertical line test, therefore it is a function.
Hence, the correct option is D.