Which of the following is a function?
a. {(1,1), (2,5), (-2,5), (1,-1)}
b. {(-1,1), (-2,1), (-1,3), (-4,1)}
c. {(-1,1), (-1,2), (-1,3), (-1,4)}
d. {(-1,1), (-2,2), (-3,3), (-4,4)}

D

Step-by-step explanation:

Reminder:  A function returns ONLY ONE y value for each input value.  

We immediately discard possible answer A, since 2 points have input 1 with different outputs.

Same for B:  discard this.

Same for C:  discard this.

D is a function.  All of the inputs are unique (no duplication).