Why can deposition increase when the Ganga moves from mountain course to plain course?

Correct answer: Because river velocity decreases as slope reduces

A river’s ability to carry sediment depends greatly on its speed and the slope of its channel. In the mountainous course, the Ganga flows down a steep gradient, so its current is generally strong and it can erode and transport large amounts of material. When it reaches the plains, the gradient becomes gentler. The river’s velocity and transporting power may decrease, especially where the channel widens or becomes less confined. Sediment is then more likely to settle on the bed or banks.

Option A is correct because a reduction in slope can reduce river velocity and promote deposition. The river may deposit sand, silt, and other alluvium, helping to form floodplains and other depositional features. The river does not need to receive water from the sea for this process, and it does not lack sediment. It also does not become a glacier. The essential link is lower slope → lower velocity → greater deposition.