What should be ensured about the DEF tank and cap during a pre-trip inspection?

During a pre-trip inspection, it is essential to ensure that the DEF (Diesel Exhaust Fluid) tank and its cap are in good condition. The correct answer underscores that the DEF tank must not leak, which is crucial since any leakage could lead to improper exhaust system function and potential damage to the environment due to fluid spillage.

Furthermore, the rubber gasket on the cap plays a critical role in preventing leaks by providing a secure seal. If the gasket is dry rotted or cracked, it can fail to maintain that seal, resulting in leaks or contamination of the DEF with dirt or moisture. A properly functioning DEF system is vital for the vehicle's emissions control, and ensuring integrity in both the tank and cap is crucial for proper vehicle operation and compliance with emissions regulations.

Other options suggest conditions that would compromise functionality, such as leaking or having damaged gaskets, which would not be acceptable during an inspection.