Which relation is a function?

{(4, 2),(3, 3),(2, 4),(3, 2)}
{(1, 4),(2, 3),(3, 2),(4, 1)}
{(1, 2),(2, 3),(3, 2),(2, 1)} ​
{(1, −1),(−2, 2),(−1, 2),(1, −2)}