Recall that for a function to be valid, each input (x-value) must give exactly one unique output (y-value). If we find that any x-value gives more than one y-value, then it is not a function.
Option 1: x = 3 gives y = 2 and y = -2 (not a function)
Option 2: x = 5 gives y = 2 and y = 8 (not a function)
Option 3: x = 3 gives y = 4 and y = 5 (not a function)
Option 4: each x-value gives only one possible y-value (this is a function)
