If the listed answers are
A: x=6
B: y=6
C: y=x
D: y=2
Then I believe your answer is x=6, or A.
The line of reflection is a line which divides the graph so that both sides essentially represent a mirror; so both sides should look like they’re reflected across one point. This point is your line of reflection, in this problem you simply have to look for where that line would be, draw a line straight down; you go through 6 on the X axis. Usually Y=[number] is a line going across whereas X=[number] is a line going up and down so from there you know it has to be the up-and-down line, x.