A river training work is generally required when the river is A. aggrading type B. degrading type C. meandering type D. both (A) and (B)

The correct answer is: A. aggrading type.

A river training work is generally required when the river is aggrading type. Aggrading rivers are rivers that are depositing sediment, which can cause the river to widen and become shallower. This can lead to flooding, erosion, and other problems. River training works can help to stabilize the river and prevent these problems from occurring.

A degrading river is a river that is eroding its bed and banks. This can cause the river to become narrower and deeper. Degrading rivers can also lead to flooding, erosion, and other problems. River training works can help to stabilize the river and prevent these problems from occurring.

A meandering river is a river that flows in a winding course. Meandering rivers can cause problems with flooding and erosion. River training works can help to straighten the river and prevent these problems from occurring.

In conclusion, the correct answer is: A. aggrading type.