The correct answer is B) put the cap on the drain. The reasoning is that after boarding the open lifeboat, the first priority is to ensure the boat remains watertight. Putting the cap on the drain plug prevents water from entering the lifeboat, which could compromise its buoyancy and stability. This is a critical first step in preparing the lifeboat for launch and ensuring the safety of those aboard. The other answer choices are incorrect because they do not address the immediate need to secure the lifeboat against taking on water. Lifting the brake handle, releasing tricing pendants, and releasing the gripes are important steps, but they should be performed after the drain is sealed.