The correct answer is: a) Southwest monsoon.
The Southwest monsoon is a seasonal wind that blows from the Indian Ocean to the Indian subcontinent from June to September. It is the main source of rainfall for Tamil Nadu. The Northeast monsoon is a seasonal wind that blows from the Bay of Bengal to the Indian subcontinent from October to December. It is a secondary source of rainfall for Tamil Nadu. Western disturbances are mid-latitude extratropical cyclones that form over the Mediterranean Sea and move eastwards across the Indian subcontinent. They are a minor source of rainfall for Tamil Nadu.
The Southwest monsoon is caused by the difference in temperature between the Indian Ocean and the Indian subcontinent. The Indian Ocean is warmer than the Indian subcontinent in the summer, which causes the air to rise over the ocean. This creates an area of low pressure over the ocean. The air then flows from the high pressure areas over the Indian subcontinent to the low pressure area over the ocean. This flow of air is the Southwest monsoon.
The Northeast monsoon is caused by the difference in temperature between the Indian subcontinent and the Bay of Bengal. The Indian subcontinent is cooler than the Bay of Bengal in the winter, which causes the air to sink over the Indian subcontinent. This creates an area of high pressure over the Indian subcontinent. The air then flows from the low pressure areas over the Bay of Bengal to the high pressure area over the Indian subcontinent. This flow of air is the Northeast monsoon.
Western disturbances are caused by the interaction of the Indian subcontinent with the Westerlies. The Westerlies are a belt of strong winds that blow from west to east in the mid-latitudes. When the Westerlies interact with the Indian subcontinent, they cause the formation of mid-latitude extratropical cyclones. These cyclones are called western disturbances in India.
The Southwest monsoon is the main source of rainfall for Tamil Nadu. It brings about 70% of the annual rainfall to the state. The Northeast monsoon brings about 20% of the annual rainfall to the state. Western disturbances bring about 10% of the annual rainfall to the state.