Which relation is a function?

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

Question

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

***[isuru]*** · Answer

hi,
 u must be familiar with vertical line test...
A curve in the xy - plane is the graph of a function x if and only if no vertical line intersects the curve more than once...

according to that answer is 2nd one