Greetings.
The answer is 7.
Step-by-step explanation:
- What is Function? - 
Function is a set of relation with domains that are not repetitive. What I'm trying to say is that in a single set, there cannot be same domain values. 
For example,
(1,2) (1,3) 
The relation above is not a function since the domain is repetitive (There are two 1's as domain which is not a function.)
But If it is (1,2) (2,1) (3,1) Then It's a function. Because there are no repetitive domains.
The question asks you which value would give the whole set only a relation and not a function.
As explained, 2 repetitive, same domains don't make the relation a function, but rather a relation instead.
Hope this helps! :)
Feel free to ask anything in the comment/reply if you are curious/confused with Function.