The correct option is Southwards in response to the southward march of the sun..
Explanation
The position of the low-pressure trough (monsoon trough) over the Indian subcontinent is governed by the apparent movement of the Sun and the associated shift of the Inter-Tropical Convergence Zone (ITCZ).
Detailed Analysis:
- Southward Movement: During the months of October and November, the apparent path of the Sun moves towards the Southern Hemisphere. In response to this southward march of the Sun, the low-pressure trough over the Ganga plains weakens and gradually shifts southwards towards the Bay of Bengal. This shift initiates the retreating monsoon season.
- Incorrect Options:
- Northwards: The trough shifts northwards towards the foothills of the Himalayas during the peak of the Southwest Monsoon (July-August), causing break monsoon conditions, not during the retreat.
- Westwards/Eastwards: The primary displacement of the pressure belts is latitudinal (North-South) following the solar cycle, rather than longitudinal.
Key Takeaway: The retreating monsoon is driven by the southward migration of the low-pressure trough from the northern plains, caused by the seasonal southward movement of the Sun.