Which relation is a function? 

As given in the question,

the relations A , B ,C and D are set up as a table of ordered pairs.

We now test to see if each element in the domain(x-values) is matched with exactly one element in the range(y-values). If so, you have a function!

A is not a function as the domain value 0 is paired with both 3 and 4.

B is a function as for each input there is exactly one output.

C is not a function as the domain value 2 is paired with both 1 and 8.

A is not a function as the domain value 1 is paired with both -3 and 2.