The correct answer is option 3, India. Thorium is a naturally occurring radioactive element that is used as a fuel for nuclear power plants. India has the largest reserves of thorium in the world. These reserves are primarily found in the coastal areas of the southern states of India, such as Kerala and Tamil Nadu.

While China, the USA, and France are also important producers of thorium, they do not have as large of reserves as India. It is worth noting that thorium reserves can vary over time as new deposits are discovered and extraction techniques improve. However, as of now, India holds the largest known reserves of thorium.