Explanation:

The Ganga river basin is the largest river basin in India, covering an area of approximately $8.6 \times 10^5 \text{ km}^2$ within the country. It supports a vast population and extensive agricultural land. Therefore, option B is correct. Option A, the Narmada, has a significant basin but is much smaller than the Ganga basin. Option C, the Brahmaputra, has a very large basin overall, but a substantial portion of it lies outside India (in Tibet and Bangladesh), making its basin within India smaller than the Ganga's. Option D, the Godavari, has the largest river basin among the peninsular rivers but is still smaller than the Ganga basin. Thus, A, C, and D are incorrect as they do not represent the largest river basin *in India*.