The correct option is A further
The given sentence is of the comparative degree as when the sentence is expanded, it means that 'He has more to travel than he has traveled till now'. If the sentence was of the superlative degree, 'the' should have preceded the adjective and it hasn't in the given sentence. The comparatives 'further' and 'farther' are often confused with each other. They both mean 'by a greater distance'. 'Further' is a much more common word, which is used for physical as well as in abstract and metaphorical contexts, and 'farther' is used when referring to physical distance. The given sentence suggests a goal 'he' has to reach, 'destination' is a metaphorical term and not a physical destination. 'Further' refers to the figurative distance, hence option A is correct and option B is incorrect. Options C and D are incorrect because they're both superlatives, when the sentence isn't of the superlative degree.