Answer:
The answer to your question is the third option
Explanation:
a) This first option is not correct because if a number is less or equal to zero does not affect in if it is a function or not.
b) It option is incorrect because each value of x has two values or y and the defition of function says that x must have only one y value.
c) This is the definition of function, it's correct
d) This option is wrong, is similar to the first option.