Question

Which relation is a function?
O {(1, 2); (2, 3); (3, 4); (2, 5)}
O {(1, 2); (1, 3); (1, 4); (1, 5))
O {(1, 2); (3, 2); (3, 3); (4, 2)}
O {(1, 2); (2, 3): (3, 4); (4, 5))