Which of the following rivers flows in a rift valley ?

Concept: A rift valley (graben) is an elongated trough formed when a block of the Earth's crust subsides between two roughly parallel fault lines, bounded by highlands on either side -- a tectonic origin distinct from an ordinary valley shaped purely by surface erosion.

Application: Among Peninsular India's rivers, the Narmada is the standard textbook example of a rift-valley river. It rises at Amarkantak in the Maikal Hills and follows a fault-bounded trough lying between the Vindhya Range to the north and the Satpura Range to the south for about 1,312 km before draining into the Gulf of Khambhat. This westward course, cutting across the general eastward slope followed by most Peninsular rivers, is a direct consequence of the rifting.

Cross-check: The Tapi River follows a parallel rift-valley course further south (between the Satpura and the Satmala hills), confirming the same tectonic pattern -- Narmada and Tapi are the two rivers conventionally cited in NCERT geography as India's rift-valley rivers. Luni is an inland-draining desert river with no rift association; Ravi and Jhelum are Himalaya-fed tributaries of the Indus system and are not conventionally classified as rift-valley rivers. This confirms Narmada as the correct answer.