The correct answer is C) must be submerged to a certain depth to release automatically. Hydrostatic release mechanisms on liferafts are designed to automatically release the raft from its cradle when submerged to a specific depth, typically around 4-6 feet of water. This ensures the raft will float free and inflate if the vessel sinks. The hydrostatic pressure at that depth triggers the release mechanism, allowing the raft to inflate and rise to the surface. The other options are incorrect - A) is not required, B) is not necessary for the hydrostatic release to function, and D) would cause the raft to prematurely inflate while still in its cradle.