What action is required if the brake warning lamp activates during a test?

When the brake warning lamp activates during a safety inspection, it indicates a potential issue with the vehicle's brake system, which is a critical component for safe operation. The warning could be due to various reasons, such as low brake fluid level, worn brake pads, or malfunctioning components.

In the context of a safety inspection, the primary responsibility is to ensure that the vehicle meets safety standards. The illumination of the brake warning lamp signifies that there is an existing safety concern, and therefore, it is necessary to reject the vehicle until the issue is resolved. This action prioritizes the safety of the vehicle's occupants and others on the road.

The other choices—such as increasing brake fluid levels or replacing brake pads—might address some underlying issues but do not directly respond to the immediate failure indicated by the warning lamp. Marking the vehicle as passed would be inappropriate since it does not rectify the safety concern. Thus, rejecting the vehicle is the correct action to ensure safety compliance.