The correct answer is Son. The Son River is a major right-bank tributary of the Ganga, originating in the Amarkantak region and joining the Ganga directly near Patna in Bihar. The Son is one of the longest rivers in central India and an important tributary system of the Ganga. Chambal, Betwa, and Ken are all tributaries of the Yamuna River, which itself is a major tributary of the Ganga. They do not join the Ganga directly.