Answer:
C. a threatening force
Step-by-step explanation:
In the context of the passage, the word "monster" is used to describe the Nautilus, which is not a mythical creature but rather a technologically advanced submarine. The connotative meaning of "monster" here leans toward a threatening force or a formidable entity created by humans (in this case, a man-made submarine). Option C, "a threatening force," best captures the intended connotation in the context of the passage.