The correct answer is option 3: Western disturbances. Western disturbances are a type of extratropical storm system that originate in the Mediterranean Sea and travel towards the Indian subcontinent. These disturbances bring winter rainfall to the north-western parts of India.
Option 1: Cyclonic depression can cause rainfall in different parts of India, but it is not the specific cause for rainfall in the north-western parts during winter.
Option 2: Retreating monsoon refers to the withdrawal of the monsoon rains from the Indian subcontinent, which occurs in the months of September and October. It does not directly cause rainfall in the north-western parts during winter.
Option 4: The South West monsoon is the primary rainy season in India, which occurs from June to September. It does not cause rainfall in the north-western parts during winter.
Therefore, the correct answer is option 3: Western disturbances, as they are the main cause of winter rainfall in the north-western parts of India.