The driver of the car being passed must _________ until the pass is complete.

The driver of the car being passed must not increase speed until the pass is complete because it's crucial for maintaining safety on the road. When another vehicle is attempting to pass, the driver allows that vehicle to complete the maneuver safely by maintaining a constant speed instead of accelerating. This reduces the potential for a collision and enables the passing driver to have enough space and time to complete the pass without any additional obstacles.

Success in passing maneuvers relies heavily on predictable driver behavior; therefore, if the driver being passed were to speed up, it would create complications for the passing vehicle, potentially leading to dangerous situations.

The other options involve actions that are either unnecessary or disruptive to the flow of traffic during a passing situation. Signaling intent to pass is the responsibility of the driver who is trying to pass rather than the one being passed. Moving to the shoulder is generally not appropriate unless directed to do so in a specific situation, and changing lanes can create confusion if not done in conjunction with proper signaling and caution.