The correct answer is Millets.
Millets are a group of cereal crops that are drought-tolerant and can be grown in dry, arid regions. They are an important source of food for millions of people around the world, and are particularly important in India, where they are a staple food for many people. Millets are also a good source of fiber, protein, and minerals.
Sugarcane is a tropical plant that is grown for its sugar. It is not drought-tolerant and requires a lot of water to grow.
Paddy is a type of rice that is grown in flooded fields. It is not drought-tolerant and requires a lot of water to grow.
Cashew nut is a tropical fruit that is grown on a tree. It is not drought-tolerant and requires a lot of water to grow.
In conclusion, millets are the most important crop for food security in Tamil Nadu due to their drought tolerance.