In which of the following rivers. The upper course contains fresh water but saline water flowing at the lower part?

The answer is the Luni river. It rises in the Aravallis near Pushkar in Ajmer district and flows south-west through Rajasthan. Its upper course, around Ajmer and Pali, carries fresh water. As it enters the arid western belt through Barmer and Jalore, the water becomes increasingly brackish, and the name itself reflects this, since Luni comes from the Sanskrit Lavanavari, meaning salty river.

• Barak (A): Rises in the hills of Manipur, flows through the Barak valley of Assam and joins the Meghna in Bangladesh. It has no saline stretch.
• Ghaggar (C): A seasonal river from the Shivaliks that flows through Haryana, Punjab and Rajasthan and gets lost in the sands. The point here is its disappearance, not a fresh-to-saline change along its course.
• D: Wrong, because Luni fits the description.

The core concept is how drainage and evaporation in an arid tract raise salinity downstream. For Mains GS-1 (arid zone geography) and GS-3 (water scarcity, desertification), Luni is a good case study of how Rajasthan's rivers behave.

Trap: Aspirants often pick Ghaggar because it is a desert river that vanishes. The question asks about salinity changing along the course, and that points to Luni.