An apparatus equipped with Selective Catalyst Reductant (SCR) will have a tank that must be filled with Diesel Exhaust Fluid (DEF) which should be:

The correct choice is based on the operational requirements of apparatus equipped with Selective Catalyst Reduction (SCR) systems. Diesel Exhaust Fluid (DEF) is crucial for SCR technology to function effectively, as it helps reduce harmful nitrogen oxide (NOx) emissions from diesel engines.

To ensure that the SCR system operates efficiently and that the vehicle remains compliant with emissions standards, it is essential to maintain an adequate level of DEF in the tank. Topping off the DEF tank every time the apparatus is fueled ensures that there is always enough fluid available for the SCR system, preventing potential engine derates or shutdowns that might occur if the DEF level drops too low. This practice promotes consistent performance of the emission control system and minimizes the risk of running out of DEF, which can lead to operational challenges.

By filling the DEF tank with each refueling, operators can integrate this task into their routine fueling process, thereby enhancing the reliability and efficiency of the vehicle. This proactive maintenance approach is critical for emergency vehicles that may require immediate and continuous operation.