C - The relationship represents a function because each input gives one unique output.
Step-by-step explanation:
To see if it is a function we can do the line test where we have a vertical line and "drag" it along the line. As long as it only hits the line once in every situation it is a function! We can see this passes and therefore the answer is c.
(why is it not d? you can have a non-linear line, but still have a function)
