The correct answer is letter D: A thick solid white line.
A thick solid white line generally separate lanes and indicate either a car can change lanes, if cars are not allowed to change lanes or if a lane will end. Sometimes they also can indicate cross walks, stop lines or parking stalls. But people associate this white lines with separating two lanes.
On the other side, people will associate any sign with an arrow to a "turning" signal. Therefore options A, B and C are generally associated with turning left or right lanes.