The correct answer is option 1 - Westerly depressions.

To understand why this answer is correct, let`s examine the other options.

Option 2 - Retreating monsoon: The retreating monsoon is associated with the withdrawal of moist air masses from northern India to the Bay of Bengal. While the retreating monsoon does bring some rainfall, it is not the main cause of the winter rains in north-western India.

Option 3 - Trade winds: Trade winds are generally prevalent in the tropical regions and are not directly associated with the winter rains in north-western India. They do not play a significant role in causing rainfall during this season.

Option 4 - South-westerly monsoon: The south-westerly monsoon is responsible for the rainfall during the summer months in India, but it is not the primary cause of the winter rains in north-western India.

The correct answer, option 1 - Westerly depressions, refers to the low-pressure systems that originate in the Mediterranean region and move eastward, bringing moist air and precipitation to north-western India during the winter months. These depressions interact with the cool and dry air masses coming from the north, resulting in the winter rains in this region.