The Ganga (Ganges) is the longest river entirely within India, flowing approximately 2,525 km from its source in the Gangotri Glacier to the Bay of Bengal. While the Brahmaputra is longer overall, much of its course lies outside India in Tibet and Bangladesh. The Godavari is the longest peninsular river but shorter than the Ganga. Option D 'Andhra Pradesh' is incorrectly listed as a river name.