For interstate driving, slow-moving vehicles must travel _________, except when passing.

For interstate driving, slow-moving vehicles are required to travel in the right-hand lane, except when passing. This regulation is in place to maintain smooth traffic flow and ensure that faster-moving vehicles can safely overtake slower vehicles without disruption. The right lane is typically designated for vehicles traveling at or near the speed limit, while the left lane is generally reserved for passing. Staying in the right lane allows faster drivers to maintain their speed with minimal interference, which is crucial for safety and efficiency on highways.

The other options do not align with the rules of interstate driving. The left lane is primarily for passing, and using the center lane can lead to confusion or impede traffic flow. Driving on the shoulder is not safe or permitted for normal travel conditions, and it should only be used in emergencies.