The correct answer is: A) Early cultivation practices.
Rice is a staple food in Kerala, and it has been cultivated in the region for thousands of years. The discovery of rice remains in prehistoric Kerala provides evidence of early cultivation practices in the region.
Option B, trade with Southeast Asia, is not supported by the evidence. There is no evidence that rice was traded between Kerala and Southeast Asia in prehistoric times.
Option C, influence of the Indus Valley Civilization, is also not supported by the evidence. The Indus Valley Civilization was located in what is now Pakistan and northwestern India, and there is no evidence that rice was cultivated in the Indus Valley Civilization.
Option D, dietary habits only, is also not supported by the evidence. The discovery of rice remains in prehistoric Kerala suggests that rice was not only a dietary staple in the region, but that it was also cultivated.
In conclusion, the correct answer is: A) Early cultivation practices.