A river is said to be of A. Aggrading type if it builds up its bed to a certain slope B. Degrading type if it cuts its bed to a certain slope C. Meandering type if it flows in sinuous curve D. All the above

The correct answer is D. All of the above.

A river is said to be of aggrading type if it builds up its bed to a certain slope. This happens when the river carries more sediment than it can carry away. The sediment is deposited on the river bed, which causes the river to become wider and shallower.

A river is said to be of degrading type if it cuts its bed to a certain slope. This happens when the river carries less sediment than it can carry away. The river erodes its bed, which causes the river to become narrower and deeper.

A river is said to be of meandering type if it flows in a sinuous curve. This happens when the river erodes its banks on the outside of a curve and deposits sediment on the inside of a curve. The river gradually bends more and more, until it forms a meander.

In conclusion, a river can be of aggrading, degrading, or meandering type. It can also be of all three types at the same time.