The river that originates from the Naag Pahar (Nag hills) near Ajmer in Rajasthan is the:

The Luni river rises in the Naag Pahar (Nag hills) of the Aravalli Range, in the Ajmer district of Rajasthan, at about 772 metres above sea level.

Course of the Luni:

• It flows south-west through the Thar Desert region of western Rajasthan, through Ajmer, Nagaur, Jodhpur, Pali, Barmer and Jalore districts.
• It is about 495 km long.
• Instead of reaching the sea, it ends in the marshy Rann of Kutch in Gujarat.
• It is the only major river of the Thar Desert and the largest river system of western Rajasthan.

Saline water:

Its upper course is fresh, but after Balotra in Barmer district, its water becomes salty because it passes through areas with salty soil and rocks. This is why its name comes from the Sanskrit "lavanavari", meaning "salt river".

Seasonal nature:

The Luni is a seasonal (ephemeral) river. It carries water mainly during the monsoon, and much of its course is dry for the rest of the year.

Sudden heavy rain can cause flash floods, as happened in 2006 near Barmer.

Tributaries:

• Left bank: Sukri, Mithri, Bandi, Khari, Jawai and Sagi, which come from the Aravallis.
• Right bank: Jojari.

Uses:

The Jaswant Sagar Dam near Jodhpur, built in 1892, stores its water for irrigation.

Inland drainage of Rajasthan:

Much of western Rajasthan has inland drainage, where seasonal streams end in salt lakes or depressions instead of reaching the sea. The Luni is the main exception, as it drains towards the Rann of Kutch.

Why not the other options?

• ✗The Banas and the Kali Sindh are rivers of the Yamuna system in eastern Rajasthan and Madhya Pradesh.
• ✗The Sabarmati rises in the Aravallis but flows into Gujarat and the Gulf of Khambhat.