In India, during October-November, with the apparent movement of the sun towards the south, the ______ over the northern plains becomes weaker.

The correct answer is B, the monsoon trough. The monsoon trough is the belt of low pressure that lies across the northern plains during the rainy season and guides the depressions that bring rain. As the sun appears to move south of the equator after the September equinox, the land cools, the low pressure weakens and the trough is replaced by high pressure. That retreat of the trough is what is called the withdrawal or retreat of the monsoon, and it begins from north-west India in early September.

Option A, an upper level trough, belongs to the upper air circulation and is not the feature described here. Option C, a lee trough, forms on the sheltered side of a mountain and is a local feature. Option D, an inverted trough, is a name used for a monsoon trough that extends into the Bay of Bengal in a particular shape, not the plains feature that weakens in October.