Answer:
a. The "I" in the passage refers to the narrator/ speaker, Professor Trilokeshwar Shonku.
b. The antonym of 'fatigue' is energized, full of vigor.
c. He compared the water to ambrosia because it tastes so good and so different from what water tasted like on earth. And ambrosia is the food of the gods in Greek mythology, capable of relieving any tiredness or weakness.
Step-by-step explanation:
"The Diary of a Space Traveler and other Stories" by Satyajit Ray tells the story of Professor Trilokeshwar Shonku and his 'space travel' stories when he suddenly disappeared. Accompanied by his companions, Newton the cat, Prahlad, his servant, and Bidhushekhar, his robot, the Professor recounts in a diary-like entry about their travel to Mars and other 'out of earth' places.
The "I" in the passage is the speaker, the professor himself.
Antonyms are the opposite words, so the antonym of "fatigue" will be "energized, vigor", etc.
The professor describes the water on Mars as like ambrosia as it tastes so good and different from what water tastes like on earth. Considering that ambrosia is the food of the gods in Greek mythology, it may be that the 'red jelly-like water' flowing tasted so good to him.