The correct answer is option B, Son.

Son River:
- The Son River is a right bank tributary of the Ganga River.
- It originates from the Amarkantak plateau in Madhya Pradesh, India.
- It flows through the states of Madhya Pradesh, Uttar Pradesh, Jharkhand, and Bihar.
- The Son River is one of the largest tributaries of the Ganga River and plays a crucial role in the economy and ecology of the region it flows through.
- It has a total length of approximately 784 kilometers.
- The Son River merges with the Ganga River near Patna, Bihar.

Other options:
a) Kosi River:
- The Kosi River is not a right bank tributary of the Ganga River.
- It is a left bank tributary of the Ganga River.
- The Kosi River originates in the Himalayas in Tibet and flows through Nepal and Bihar in India before joining the Ganga River.

c) Gomti River:
- The Gomti River is not a right bank tributary of the Ganga River.
- It is a major river in Uttar Pradesh, India.
- The Gomti River originates from the Gomat Taal in Pilibhit district of Uttar Pradesh and flows through Lucknow before joining the Ganga River near Saidpur, Uttar Pradesh.

d) Ghaghara River:
- The Ghaghara River is not a right bank tributary of the Ganga River.
- It is a major river in northern India and Nepal.
- The Ghaghara River originates from the Gangotri Glacier in Uttarakhand, India, and flows through Uttar Pradesh and Bihar before joining the Ganga River near Chhapra, Bihar.

Therefore, the correct answer is option B, Son River, which is a right bank tributary of the Ganga River.