How is a portable cleaning machine typically grounded when cleaning cargo tanks?

The correct answer reflects the practice of ensuring electrical safety and preventing static discharge during cleaning operations in cargo tanks. When using a portable cleaning machine, grounding is essential to mitigate the risk of sparks that could ignite flammable vapors present in the tanks.

The option regarding the water supply hoses containing internal wires pertains to the design of certain cleaning machines that incorporate grounding features directly into the equipment. These internal wires create a conductive path that allows static electricity to be directed safely away from the machine and into the ship's grounding system. This method effectively reduces the potential for electric shocks and fire hazards, making it an important safety measure in the hazardous environment of cargo tanks.

In contrast, while other methods of grounding might be practical, they either do not directly address the principles of electrical safety, or they depend on conditions that may not consistently provide a reliable grounding pathway. Thus, the relevance of internal wires within the hoses is crucial for maintaining a safe operation when cleaning the cargo tanks.