Question

12 questions total? Please help.
Which relation is a function?

{(1, 2); (2, 3); (3, 4); (1, 5)}
{(1, 2); (1, 3); (1, 4); (1, 5)}
{(1, 2); (2, 3); (3, 4); (2, 5)}
{(1, 2); (2, 2); (3, 2); (4, 2)}