Which condition must exist for a compartment to completely flood from the deck to the overhead?

The condition that allows a compartment to completely flood from the deck to the overhead is that it must be vented. Vents enable water to enter the space, allowing it to fill entirely from the bottom to the top. If the compartment were sealed without any vents, the air trapped inside would inhibit the flow of water and prevent complete flooding.

In contrast, securely sealing the compartment from the sea, isolating it from other spaces, or eliminating ventilation fittings may not facilitate flooding in the same manner. While these options speak to conditions that can contribute to water management and control, they do not specifically address the requirement for allowing water to completely fill a compartment. Vents are essential for accommodating changes in pressure and facilitating the entry of water, making them critical for complete flooding.