"It had no discernible pattern." is wrong, because there is a pattern, noticeably.
"It traveled chiefly by land" is also wrong, since we can see that the disease traveled by sea in the map.
"Landlocked Areas were the safest" is also wrong since there's a hardly affected coastal area.
So in the end, we must go with It followed common sea routes.
Not only it is the correct one by elimination, considering that it cannot be any of the others, the pattern of the sea routes is very noticeable in the map, you just have to follow the red arrows to see it.
