The correct answer is A. Superfluous.
A submarine is a watercraft capable of independent operation underwater. Submarines are used for military, commercial, and recreational purposes. Military submarines are used for warfare, reconnaissance, and transportation. Commercial submarines are used for tourism, research, and salvage. Recreational submarines are used for diving and exploration.
The word “superfluous” means “more than is necessary or useful.” In the context of the question, the word “superfluous” is used to describe submarines because they are able to operate below the surface of the seas, which is more than is necessary or useful for most watercraft.
The word “surreptitious” means “done in a secret or stealthy way.” In the context of the question, the word “surreptitious” would not be a good choice because submarines are not typically used in a secret or stealthy way.
The word “perspicacious” means “having or showing keen insight or discernment.” In the context of the question, the word “perspicacious” would not be a good choice because submarines do not typically require keen insight or discernment to operate.
The word “subterranean” means “situated or occurring beneath the surface of the earth.” In the context of the question, the word “subterranean” would not be a good choice because submarines are not typically situated or occurring beneath the surface of the earth.