The correct answer is D) grooved wheel in a block. A sheave is a grooved wheel, typically in a block or pulley, that allows a line or cable to run smoothly through it. Sheaves are an essential component in rigging and mechanical advantage systems used on vessels, as they reduce friction and wear on the lines. The grooves in the sheave are designed to guide and support the line, enabling it to change direction with minimal resistance. The other answer choices are incorrect because they do not accurately describe what a sheave is. A seaman's knife is a type of knife, a partial load of grain is not a mechanical component, and a line to hold a lifeboat is a different type of equipment on a vessel.