It seems like the first one. The road line leads through Kashgar and to others, but the other answers aren't apart of the road line.
Answer:
"Historians have spent years trying to crack language codes.
Explanation:
This one gives background on having to decode something meaning they are trying to figure out what the language or thing is.
Depending on the sentence you can use a general pronoun for an object like “it” “that” or “this”
