Final answer:
Material condition Yoke involves securing all non-essential watertight doors, hatches, and scuttles to prevent progressive flooding and ensure ship readiness, offering more security than condition X-Ray but less than condition Zebra.
Step-by-step explanation:
When a ship is in material condition Yoke, the classifications that are secured include all watertight doors, hatches, and scuttles that are not essential to the operation of the ship or the comfort of the crew. These are typically fittings or passageways that, if left unsecured, could allow progressive flooding.
This condition is set during wartime or when there's a threat to the security of the ship. It provides a level of readiness that is more secure than peacetime material condition X-Ray, but less than the most secure condition, Zebra, which is set when the ship is expecting an imminent attack.