The correct answer is C) Flexible shaft. The flexible shaft screwdriver is designed with a shaft made from double coil wire or tightly coiled steel, which allows the shaft to bend and flex. This flexibility makes the flexible shaft screwdriver particularly useful for working in tight spaces or accessing screws in hard-to-reach areas on a vessel. The other answer choices do not describe a screwdriver with a flexible shaft. Square shank, jeweler's, and ratchet screwdrivers have different shaft designs that are not specifically flexible.