The question asks which one of the following is not a tributary of the river Ganga. To determine the correct answer, we need to understand the definition of a tributary. A tributary is a smaller river or stream that flows into a larger river, known as the main river.

Option 1: Son - The Son River is indeed a tributary of the river Ganga. It originates in Madhya Pradesh and flows through the states of Chhattisgarh, Jharkhand, and Bihar before joining the Ganga in Patna.

Option 2: Mahananda - The Mahananda River is also a tributary of the Ganga. It originates in the Himalayas of Nepal and flows through the Indian state of West Bengal before joining the Ganga in the city of Farakka.

Option 3: Teesta - The Teesta River is not a tributary of the Ganga. It originates in the Himalayas of Sikkim and flows through the Indian states of Sikkim and West Bengal. It ultimately flows into the Brahmaputra River in Bangladesh.

Option 4: Sharada - The Sharada River, also known as the Kali River, is not a tributary of