The river mentioned most frequently in the Rig Vedic hymns was:

In the Rig Veda, the Sindhu (Indus) river is the most frequently mentioned river due to its supreme geographical importance to the early Vedic people. However, looking closely at the provided options, the Saraswati holds the distinction of being the most sacred and highly revered river (Naditama - best of all rivers) in the text, and it is explicitly celebrated across various hymns. Since Sindhu is absent from the choices, Saraswati is the correct response here.