Answer: Choice (3)
Explanation:
For choice (3), the input C maps to more than one output (The values 1 and 3 at the same time). A function must have each input map to one and only one output. So this is why relation (3) is not a function.
The other answer choices (1), (2) and (4) have each input point to exactly one output. So these are functions that can be ruled out.
