The correct answer is A) Cut the painter. After everyone is safely onboard the inflatable life raft, the first action should be to cut the painter (the line securing the raft to the sinking vessel). This is crucial to ensure the life raft drifts away from the sinking ship, preventing it from being pulled under or damaged as the vessel goes down. The other options are not the correct first step. Operating the radio equipment (B) and ventilating the raft (C) are important tasks, but they should be addressed after the raft has been safely separated from the sinking vessel. Opening the equipment pack (D) may also be necessary, but it is not the most critical immediate action.