Religions expanded along trade routes primarily because of option A: merchants and traders exchanged ideas as well as products.
Trade routes facilitated interactions among diverse cultures, allowing for the exchange not only of goods but also of beliefs and ideas. As merchants traveled and established networks, they often shared their religious beliefs and practices with one another and with the communities they encountered.
While options B, C, and D may have some historical contexts, they do not encapsulate the broader and more significant dynamic of how religions spread along trade routes. For instance, war (B) might force conversions but is not a primary reason for the general spread of religions along trade networks. The economic advantages of trading as a member of a particular faith (C) or coercive measures (D) while relevant in certain instances, do not explain the voluntary and organic nature of religious exchange facilitated by commerce in many historical contexts.
Therefore, the primary reason for the expansion of religions along trade routes is the exchange of ideas by merchants and traders.