Which equation is a line of symmetry for the graph shown

For a line, a symmetric line is any perpendicular line to it, because if you drop a perpendicular line, the line on one side is just a reflection of the other side.

so, in short, what is a likely equation of a line perpendicular to that one?

well, what's the slope of the line on the graph anyway?  well, notice, we have a point of (-3,-4), let's pick another point on the line to get its slope, hmmm say (0,8),





so, a line whose slope is -1/4 is a likely line of symmetry for that line, since it'd be a perpendicular line to it, and since you know your slope-intercept forms, you know which one that is.