Answer: the correct answer is farther
Explanation: farther is the comparative form of far. There is another comparative form of the adverb far which is further.
"Further" Versus "Farther" The quick and dirty tip is to use “farther” for physical distance and “further” for metaphorical, or figurative distance. It's easy to remember because “farther” has the word “far” in it, and “far” obviously relates to physical distance.