Which relation is a function?
Question 3 options:

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

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

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

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