Which relation is a function?

{(0, –9), (–9, –2), (0, –3)}

{(0, –9), (–2, –3), (–2, 0), (–3, –2)}

{0, –9, –2, –3}

{(0, –9), (–9, 0), (–3, –3)}

The only relation that is a function is {(0, –9), (–9, –2), (0, –3)}. This is because each input (x-value) is only associated with one unique output (y-value).