The Vedic river 'Sadanira', which marked the boundary between the Kosala and Videha regions, is identified with the modern:

The river "Sadanira", mentioned in the Shatapatha Brahmana, is identified with the present-day Gandak (Gandaki) river.

About Sadanira in the Shatapatha Brahmana:

• "Sadanira" means "always full of water" (perennial).
• The Shatapatha Brahmana tells the story of Videgha Mathava, a king, and his priest Gotama Rahugana, who followed Agni (fire) as it moved eastwards from the banks of the Saraswati, burning the land.
• Agni stopped at the river Sadanira, which it did not cross.
• Videgha Mathava then crossed the river and settled to its east, founding the land of Videha (north Bihar).
• The river formed the boundary between Kosala (to the west) and Videha (to the east).

What the story tells us:

• It shows the eastward movement of the Vedic people in the later Vedic period, from the Saraswati region to the Ganga valley and Bihar.
• The burning of the land by Agni may symbolise the clearing of forests by fire for agriculture.
• It explains how the marshy land east of the Sadanira was settled and brought under cultivation.

About the Gandak:

• It rises in the Himalayas in Nepal (as the Kali Gandaki).
• It enters India in Bihar, near Valmikinagar, and joins the Ganga near Patna (at Hajipur).
• It is known for "shaligram" stones, fossil ammonites found in the Kali Gandaki, which are worshipped as symbols of Vishnu.
• The Gandak Project, an irrigation project, is a joint project of India and Nepal.

Videha:

• Its capital was Mithila (identified with Janakpur in Nepal).
• It was ruled by King Janaka, father of Sita, and a patron of philosophers such as Yajnavalkya.
• It later became part of the Vajji confederacy.

Why not the other options?

• ✗The Ghaghara (Saryu) flows through Ayodhya.
• ✗The Son is a southern tributary of the Ganga.
• ✗The Kosi is called the "Sorrow of Bihar".