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

A relation is a function if & only if for each value of x we have (at the most) one & only one value of y.
In short if we have a repeat value of x that correspond to 2 different values of y then it's not a function but a relation instead

Let's take the 1st example:
    x     I   y
I
1        I  2
3        I  2
3        I  3
4        I  2
We notice that there are 2 SAME VALUE of x (that is 3) that have various value of y, Hence this is not a function. Mind you this concerns x ONLY.
In short a function doesn't allow a repeat of same x.
Follow this methodology & you will find the other problems