Parallel. This is pretty simple if you either a) Picture it in your head, or b) look at each option logically. lets try out b)
The last option does not really apply to lines, and is for angles. (unless your talking about the adjecent sides of an angle, but this would mean they meet at a vertex) Perpendicular, doesnt make sense because if they were perpindicular on a plane, (2-d) then it would be immpossible for BOTH to be parallel to a third line. This leaves the first option, which makes sense.
