Through these roads and ________ eagerly streamed the advance agents of civilization.

This sentence highlights a very general statement that almost all the civilization of the world advanced on roads and near water bodies. 'Cascades' is waterfall, hence ruled out. 'Maunders' is to move or act in dreamy, vague way, hence irrelevant in this context. “Water courses” is more general and broad term and includes all types of water bodies, instead of tributaries, which is more specific and 'aqueduct' that is a large pipe or conduit made for bringing water from a distant source. Hence, (3) is the answer.