Concept:
The Son (Sone) River rises near Amarkantak in Madhya Pradesh and flows through Madhya Pradesh, Chhattisgarh, Uttar Pradesh, Jharkhand and Bihar before joining a larger river.
Explanation:
The Son River merges with the Ganga near Danapur, close to Patna in Bihar, after a course of roughly 780 kilometres, making it one of the Ganga's major right bank tributaries. Option B, Yamuna, is itself a separate tributary of the Ganga and does not receive the Son. Option C, Narmada, flows independently westward into the Arabian Sea and is unrelated to the Son. Option D, Mahanadi, is a separate river system draining into the Bay of Bengal through Odisha, again with no connection to the Son. Only the Ganga is the river that the Son actually joins.
Final Answer:
The Son river is a tributary of the Ganga, option A.