Not a function because 4 has two outputs, 3 and 4.
1) Examining that diagram we can see that
The entry 4 in the Domain has two corresponding values.
And because of that, this cannot be taken as a function, since each entry in the Domain (diagram to the left) must correspond to one and only output on the right.
2) So, examining the options the best one is
"Not a function because 4 has two outputs, 3 and 4."
