Explanation:

- Godavari is the longest river in Peninsular India, often referred to as the "Dakshin Ganga" or "Ganga of the South".

- It flows for about 1,465 kilometers and discharges into the Bay of Bengal.

- Originating from the Western Ghats in Maharashtra, it flows through several states and provides irrigation and water resources.

- Krishna is another major river, stretching around 1,290 kilometers, making it the second longest in the region.

- Narmada runs westward between the Vindhya and Satpura ranges, but it is shorter than the Godavari.

- Luni is a seasonal river in Rajasthan, much shorter and less significant in comparison.

Godavari is the correct answer.