Question

Which of the following sets represents a function?
{(1, 1), (2,1), (3,1), (4, 1)}
{(1, 1), (1,2), (1,3), (1, 4)} {(1, 1), (2,1), (2,2), (3, 1)} {(2, 1), (2,2), (3,2), (4, 2)}