Option 1: Western disturbances are low-pressure systems that form over the Mediterranean Sea and move towards India. These disturbances bring rainfall during winters in the north-western part of India. They are responsible for the formation of rain-bearing clouds and precipitation in the region.

Option 2: Cyclonic depressions are also low-pressure systems, but they are more common during the summer months and are associated with the monsoon season. They are not directly responsible for rainfall during winters in the north-western part of India.

Option 3: Southwest monsoon is responsible for bringing rainfall to most parts of India during the summer months. It is not the main cause of rainfall during winters in the north-western part of India.

Option 4: Retreating monsoon refers to the withdrawal of the southwest monsoon from the northern parts of India. It is not the main cause of rainfall during winters in the north-western part of India.

Alert - correct answer should be option 1: Western disturbances.