The correct answer is: A. Bagmati
The Bagmati River is a major river in Nepal and India. It is the largest tributary of the Ganges River. The Bagmati River originates in the Himalayas and flows through the Kathmandu Valley before joining the Ganges River in India. The Bagmati River is a sacred river for Hindus and is considered to be one of the most important rivers in Nepal.
The main tributaries of the Bagmati River are the Balason, Ratwa, and Kankai rivers. The Balason River is a tributary of the Bagmati River that originates in the Mahabharat Range. The Ratwa River is a tributary of the Bagmati River that originates in the Chure Range. The Kankai River is a tributary of the Bagmati River that originates in the Himalayas.
The Bagmati River is a major source of water for irrigation and drinking water in Nepal. The river is also a popular destination for fishing and boating. The Bagmati River is a vital part of the ecosystem of Nepal and is an important source of livelihood for many people.
The other options are incorrect because they are not tributaries of the Bagmati River. The Son River is a tributary of the Ganges River in India. The Mahananda River is a tributary of the Brahmaputra River in India. The Gandak River is a tributary of the Ganges River in India.