Functions
A function is a relation in which each input has only one output.
If we are given a set of ordered pairs (x,y) and find that for a specific value of x there is more than one value of y, then it's NOT a function.
Let's analyze each option.
The first set contains two ordered pairs (7,-3) and (7,9) that give for x=7 two different values of y, so this is not a function.
The third set contains the ordered pairs (6,6) and (6,-7). The input value 6 has two different output values. This is not a function either.
The fourth set also fails to be a function because (0,-5) and (0,-2) give to x=0 two different outputs for y.
The second option is a function because every value of the input set has only one value in the output set.