PLEASE HELP!!!!!!!!!!!!!

Which of the following relationships represents a function?
A.
x -1 6 2 6
y 15 13 15 14


B.
x -1 6 -1 10
y 15 15 21 14


C.
x -1 6 1 10
y 15 13 21 15


D.
x -1 6 1 1
y 15 13 15 14