If the upward buoyancy and displacement weight of a submarine typically equalizes with two-thirds of the hull submerged, how does it sink?

The correct answer is based on the principle of buoyancy and the interaction between weight and volume regarding the submarine's ability to submerge or rise in water.

When a submarine wants to descend, it must increase its displacement weight. This is accomplished by taking on water into its ballast tanks. As the submarine fills these tanks with water, the overall weight of the submarine increases. This increase in weight surpasses the upward buoyant force acting on the submarine, which is derived from the water it displaces.

When the downward force (displacement weight) becomes greater than the upward buoyant force, the submarine sinks. This process allows the submarine to control its depth effectively. The submarine can thus achieve greater depth by having more of its hull submerged, typically compensating for the influences of buoyancy until the equilibrium is re-established once again.

The other choices suggest alternative mechanisms which do not accurately reflect the principles in play during the process of submersion. Increased weight via taking on water is how submarines effectively manage their buoyancy and depth, making the correct answer align with this fundamental principle of physics.