Your anser would be A) function, an input has one output. Let me explain it now.
Functions are relations that derive one output for each input, or that is the technical definition for it, or one Y value for any X value inserted into the equation. A function is made by a set of ordered pairs. examples of ordered pairs are (4,7) as 4 is the X value and Y is the Y value.
Its quite simple to see if a function is or is not a function. Here is an example
[(4,7),(6,9),(4,1),(9,0)] is not a function because when you look at all the x values which are 4,6,4, and 9, you see that the #4 is repeated signalling that It it not a function.
[(2,3),(9,3),(1,6),(8,2)] is a function, lets check it. All the x values are 2,9,1, and 8, and as you can tell, none of them are the same.