well your domain is your x and the range is your y
the relation is a function if
1) x has a corresponding y
2) any one x has only one y
think about when you graph a point (x, y) if the same x has two different ys' then the graph will NOT be a function because does not pass the vertical line test. Like (2,4) and (2,-4) could be touched by a vertical line.
A. is NOT a function because x=6 has multiple y=9, y=10, y=11
B. is a function because x=4 has y=6, x=7 has y=3, x=9 has y=7!!!
C. is NOT not a function because x=5 has two y, y=6 and y=9
D. is NOTa function because x=5 has two y, y=1 and y=8