The correct options are
A They were the entry points for the subcontinent.
B The important trade locations were in these areas.
C These areas were rich in fertile lands and had ample source of water.
River valleys and coastal areas were important trade locations as they were the entry points for the country and had fertile lands and water resources leading to lots of population which in turn provided raw materials for trade. Thus the control of these regions were important for power. The natural beauty of these areas did not have any effect on trade. This was because people in ancient times mostly travelled for trade, pilgrimage or settlement, and not much for sight-seeing to places.