The correct answer is: A. presence of an excessive bed slope in the river
Meandering is a type of river flow where the river channel bends and curves. It is caused by the presence of an excessive bed slope in the river, which causes the water to flow faster on the outside of the bend and slower on the inside of the bend. This difference in velocity causes the river to erode the outside of the bend and deposit sediment on the inside of the bend, which makes the bend even sharper.
Option B, degradation, is the process of a river channel becoming deeper and wider. This can be caused by a number of factors, including the presence of an excessive bed slope, but it is not the main cause of meandering.
Option C, the extra turbulence generated by the excess of river sediment during floods, can also contribute to meandering, but it is not the main cause.
Option D, none of the above, is incorrect. The main cause of meandering is the presence of an excessive bed slope in the river.