On interstate highways, where must slower vehicles travel?

Answer: B — In the right-hand lane

Slower vehicles are required to travel in the right-hand lane on interstate highways to facilitate smoother and safer traffic flow. The main purpose of this rule is to ensure that faster-moving traffic can pass without delay. The right lane is designated for vehicles traveling at lower speeds, allowing faster vehicles, which typically use the left-hand lane, to overtake efficiently. By driving on the right, slower vehicles help reduce the chances of accidents that may occur if they were to occupy the faster traffic lanes, thus maintaining optimal traffic patterns and promoting safety on the highway. The other choices are not suitable because the left-hand lane is primarily intended for faster traffic and overtaking maneuvers. The center lane, when present, often serves a similar purpose as the left lane, especially on three-lane highways. Traveling on the shoulder is designated for emergency situations or breakdowns, not for regular vehicle operation, making it unsafe and illegal for general traffic usage.