Introduction:
Tanka is a traditional rainwater harvesting technique that is primarily associated with the state of Rajasthan in India. It is an ancient method of collecting and storing rainwater for future use, especially during the dry seasons.

Explanation:
Tanka is a unique water storage system that has been practiced for centuries in the arid regions of Rajasthan. It involves the construction of underground tanks or reservoirs to collect and store rainwater. These tanks are usually made of stone or bricks and are designed to capture and store rainwater during the monsoon season.

Key Features of Tanka:
- Underground Storage: Tanka tanks are built underground, which helps in reducing evaporation and keeping the water cool. This helps in preventing water loss and maintaining the quality of stored water.
- Filtration System: Tanka tanks are equipped with a filtration system that allows the rainwater to pass through a series of filters, removing impurities and sediment. This ensures that the stored water is clean and suitable for various purposes.
- Roof Catchment System: Tanka tanks are typically connected to rooftops through a network of channels and pipes. The rainwater falling on the roofs is directed towards the tanks, maximizing the collection of water.
- Multi-Purpose Usage: The collected rainwater in Tanka tanks can be used for various purposes such as drinking, cooking, irrigation, and livestock watering. It provides a reliable and sustainable source of water in the arid regions of Rajasthan.

Association with Rajasthan:
Rajasthan is known for its arid climate and limited water resources. The state experiences low rainfall and frequent droughts, making water scarcity a major challenge. Tanka, as a rainwater harvesting technique, has been widely practiced in Rajasthan to mitigate these water-related issues.

The traditional knowledge and expertise in building and maintaining Tanka tanks are passed down through generations in Rajasthan. The state's geographical and climatic conditions make it an ideal region for the implementation of Tanka systems. The technique has proved to be highly effective in conserving water and meeting the water demands of the local communities in Rajasthan.

Therefore, the rainwater harvesting technique of Tanka is primarily associated with the state of Rajasthan.

Explanation:
The rainwater harvesting technique called Tanka is associated with the state of Rajasthan.
Reasoning:
Tanka is a traditional rainwater harvesting technique that is commonly used in the arid regions of Rajasthan. It involves constructing underground tanks or reservoirs to collect and store rainwater during the monsoon season. The harvested rainwater is then used for various purposes such as drinking, cooking, and irrigation throughout the year.
Conclusion:
Therefore, the correct answer is option D: Rajasthan.