Final Answer:
The correspondent encountered the fin of a shark in the ocean (Option 2).
Step-by-step explanation:
In this passage, the description of a long, loud swishing astern of the boat, accompanied by a gleaming trail of phosphorescence resembling blue flame, suggests a marine creature's presence. The mention of a trail that "might have been made by a monstrous knife" aligns with the characteristics of a shark's fin cutting through the water. This imagery strongly supports the conclusion that the correspondent witnessed the fin of a shark in the ocean.
The narrative's focus on the phosphorescent trail and its comparison to a monstrous knife eliminates options such as a mermaid's tail or a hallucination with no meaning. The specificity of the description, combined with the correspondent's heightened senses in the nighttime setting, points to a tangible and potentially threatening encounter with a shark. This interpretation aligns with the context of the story and the inherent dangers associated with life at sea.
In conclusion, the textual evidence and contextual analysis lead to the confident identification of the sight as the fin of a shark. This encounter adds a layer of tension and peril to the narrative, emphasizing the unpredictable challenges faced by the characters in the open ocean during their journey.
Correct option is (Option 2).