Which of the following must be checked when preflighting the portable water fire extinguisher?

When preflighting the portable water fire extinguisher, it is essential to check that the wire seal is intact and there is no corrosion present. The wire seal serves as a crucial indicator that the extinguisher has not been tampered with, ensuring that it is ready for use in case of an emergency. Additionally, the presence of corrosion can compromise the integrity and operational efficiency of the extinguisher. An intact wire seal and the absence of corrosion assure that the unit is secure and reliable for effective use in extinguishing fires.

The other options involve checks that are either not specifically relevant for water fire extinguishers or represent misconceptions about their operational requirements. For instance, the requirement about being under 75% full does not apply, as extinguishers should typically be checked to ensure they are appropriately filled rather than underfilled. The notion that the extinguisher must always be "in the green" is more relevant to pressure indicators on certain types of extinguishers, but water extinguishers do not typically have such gauges. Lastly, inspecting only the nozzle fails to cover the comprehensive safety checks needed for the entire extinguisher, which includes verifying seals and looking for any signs of damage or wear.