Final answer:
The great fish in the passage is propelled primarily by swimming.
Step-by-step explanation:
The primary mode of propulsion for the great fish in the passage is swimming. The passage describes how the fish moves through the water using its crescent tail and pectoral fins to change direction. It mentions that the fish lacks a flotation bladder and fluttering flaps, so it must continuously swim to stay suspended in the water. Stopping would cause the fish to sink and die. Therefore, the correct answer is a. Swimming.