The correct answer is option D, Tapi, as it is not a tributary of the River Ganga. The River Ganga, also known as the Ganges, is one of the major rivers in India and is considered sacred by Hindus. It originates in the Gangotri Glacier in the state of Uttarakhand and flows through the northern part of India before emptying into the Bay of Bengal.

Below is an explanation of the other options and why they are tributaries of the River Ganga:

a) Son:
- The Son River is one of the largest tributaries of the River Ganga.
- It originates in the Amarkantak Hills of Madhya Pradesh and flows through the states of Jharkhand, Bihar, and Uttar Pradesh before joining the Ganga near Patna.
- The Son River is known for its high silt-carrying capacity and is an important tributary in terms of water discharge.

b) Ghaghra:
- The Ghaghra River, also known as the Saryu River, is a major tributary of the Ganga.
- It originates in the Tibetan Plateau and flows through Nepal and the Indian states of Uttarakhand, Uttar Pradesh, and Bihar before joining the Ganga near Chhapra.
- The Ghaghra River is known for its unpredictable nature and is responsible for frequent floods in the region.

c) Chambal:
- The Chambal River is another important tributary of the River Ganga.
- It originates in the Vindhya Range in Madhya Pradesh and flows through Rajasthan and Uttar Pradesh before joining the Ganga near Etawah.
- The Chambal River is known for its pristine and undisturbed ecosystem, and it is home to several endangered species such as the Gharial and the Gangetic Dolphin.

d) Tapi:
- The Tapi River, also known as the Tapti River, is not a tributary of the River Ganga.
- It is a river in western India that originates in the Satpura Range in Madhya Pradesh and flows through Maharashtra and Gujarat before emptying into the Arabian Sea.
- The Tapi River is not connected to the River Ganga or its basin.

In conclusion, the tributaries of the River Ganga include Son, Ghaghra, and Chambal, while Tapi is not a tributary of the River Ganga.