Which type of naval vessel is described as self-propelled and capable of underwater operations?

A naval vessel described as self-propelled and capable of underwater operations is indeed a submarine. Submarines are uniquely designed to operate beneath the surface of the water, using various technologies that allow them to navigate and carry out missions while submerged. Their self-propelled nature enables them to travel at significant speeds underwater, and they are equipped with sophisticated sonar and communication systems to navigate and engage in tactical operations while being stealthy.

This capability sets submarines apart from other types of naval vessels. For example, surface combatants, while self-propelled, operate on the surface of the water and are primarily designed for engagements above water. Similarly, patrol combatants are focused on surface actions and short-range missions, and amphibious warfare vessels are designed to transport troops and equipment from sea to shore. Unlike submarines, these other vessels have limited or no underwater operation capabilities, which is a defining characteristic of submarines. Thus, the specific ability to operate submerged confirms that the answer is a submarine.