What must inspectors check regarding the vehicle’s gas cap?

Inspectors must ensure that the vehicle's gas cap is secure and functioning properly to prevent fuel vapor leaks. This is an essential part of the emissions control system, as an ineffective or loose gas cap can lead to the release of harmful vapors into the atmosphere. A well-functioning gas cap helps maintain the integrity of the fuel system and reduces emissions, which is crucial for environmental protection and compliance with regulations. By verifying that the gas cap is not only tightened but also sealed well without damage, inspectors contribute to the overall effectiveness of the vehicle's emissions control system, ensuring that it meets the required standards.

The other options do not focus on the primary function of the gas cap related to emissions control, which is why they are not correct in this context.