The correct answer is: A) Rain shadow effect.
The Narmada River valley is located in the rain shadow of the Western Ghats. This means that the mountains block the moisture-laden winds from the Arabian Sea, causing the region to receive less rainfall than areas to the east. The result is a semi-arid climate with hot, dry summers and cool, dry winters.
Option B is incorrect because the Narmada River valley does not experience a high concentration of rainfall. In fact, it receives relatively little rainfall compared to other parts of India.
Option C is incorrect because the Narmada River valley does not experience more consistent rainfall throughout the year. The region experiences a monsoon climate, with most of the rainfall occurring during the summer months.
Option D is incorrect because the Narmada River valley does not experience lower humidity levels. In fact, the region has relatively high humidity levels, especially during the summer months.