The correct answer is (c), rainwater harvesting.
Rainwater harvesting is the collection and storage of rainwater for various uses, such as watering crops, washing clothes, and flushing toilets. It is a cost-effective way to conserve water, especially in areas that experience frequent droughts. Rainwater harvesting can also help to reduce flooding and improve water quality.
The other options are not as cost-effective as rainwater harvesting. Improvement of the flush system would require significant investment in new infrastructure, while treatment and use of wastewater would require expensive treatment plants. Recycling and reuse of water is also a viable option, but it can be more expensive than rainwater harvesting, depending on the specific circumstances.
In conclusion, rainwater harvesting is the most cost-effective water management strategy in India. It is a sustainable way to conserve water, and it can help to reduce flooding and improve water quality.