In which of the following rivers does the upper course contain fresh water while saline water flows in the lower part?

1. A

   Barak River

   The Barak is a freshwater river of the Meghna system (Manipur–Assam–Bangladesh). It does not turn saline in its Indian lower course in the sense this question means.

2. B

   Luni River

   The Luni rises in the Aravallis (near Ajmer) with relatively fresh water in its upper course and becomes increasingly saline downstream as it crosses the Rajasthan desert toward the Rann of Kachchh, never reaching the sea as a normal freshwater river. That is the official key.

3. C

   Ghaggar River

   The Ghaggar is a seasonal river of the Punjab–Haryana–Rajasthan plains (often linked to palaeo-Saraswati debates). Salinity of the Luni type is not its textbook tag here.

4. D

   None of the above

   A named river (Luni) fits, so ‘none’ is wrong.

Summary. Official key is (b) Luni. Upper Luni is fresher; the lower desert course is saline toward the Rann. Barak and Ghaggar are not this pair.