I found an explanation for this question:
The psychologists are making the following claim: that a person who FEELS PERSONALLY RESPONSIBLE will drive AT THE APPROPRIATE SPEED.
When there is no speed limit sign, each driver feels PERSONALLY RESPONSIBLE for determining the appropriate speed.
The result is that people drive at the appropriate speed.
When there is a speed limit sign, the SIGN determines the appropriate speed.
Because drivers no longer feel personally responsible for determining the appropriate speed, they drive faster.
The correct answer must strengthen the link between FEELING PERSONALLY RESPONSIBLE and DRIVING AT THE APPROPRIATE SPEED.
Answer choice B strengthens this link:
Drivers who feel personally responsible for operating their vehicles at speeds appropriate to the conditions of the road tend to drive faster when no pedestrians are present than they do when pedestrians are present.
The implication is that people drive more slowly when a pedestrian is present because they feel PERSONALLY RESPONSIBLE for the safety of the pedestrian.