C represents a function.
A function will return ONE value for each possible value to input. Remember this. ONE input = ONE output. Now let's look at the 4 options you have.
A. You can use any value of X from 0 to infinity. That's OK. But unfortunately, you're getting 2 possible return values for each of those inputs. So it's NOT a function.
B. You can only have an input of X=-2. That's also OK. But then you have an infinity number of possible Y values. So also NOT a function.
C. Looks like you can any X input from -infinity to +infinity. And for every one of those possible inputs, there's exactly 1 Y value. So this is a function.
D. X can range from 0 to +infinity. That's OK. But for every X value, there's 2 possible Y values. So not a function.
