The correct answer is C) accidental unhooking when the falls become slack. Preventer bars are fitted on lifeboat releasing gear to prevent accidental unhooking of the falls (the ropes or wires that support the lifeboat) when they become slack. This is a crucial safety feature, as unintended release of the falls could cause the lifeboat to drop unexpectedly, potentially injuring or endangering the occupants. The other options are incorrect because they do not accurately describe the primary purpose of preventer bars. Option A is incorrect as preventer bars do not prevent accidental operation of the releasing gear itself. Option B is incorrect as preventer bars are not used to prevent re-hooking of the falls. Option D is incorrect as preventer bars are not specifically used to prevent operation of the release lever until the boat is waterborne.