Answer:
C
The problem:
What can x not be so that the following is a function:
(8,5), (6, 8), (X, 11), (0, 12)?
Explanation:
A set of points is a function if every element,x, in the domain is paired with one y from the range.
So basically we don't want any repeated x. x would be a repeat if x is 8 since it has already been paired with 5.
Summary we are saying x can't be 8 because x=8 is already paired with y=5 so we can't pair it with another different y.