On interstate highways, where must slow-moving vehicles travel?

Answer: C — In the right-hand lane

Slow-moving vehicles are required to travel in the right-hand lane on interstate highways to maintain a smooth flow of traffic. The right lane is generally where vehicles that are traveling at lower speeds can safely operate without impeding faster traffic, which is typically found in the left lanes. This positioning helps prevent accidents and promotes better overall traffic management by allowing faster vehicles to pass on the left. The other lanes, such as the left-hand and middle lanes, are generally intended for vehicles traveling at higher speeds, making it unsafe and inefficient for slow-moving vehicles to occupy those spaces. Meanwhile, the shoulder lane is not meant for regular driving; it's specifically designated for emergencies and breakdowns, further emphasizing the necessity for slow-moving vehicles to remain in the right-hand lane for safety and operational effectiveness.