If there are two canals taking off from each flank of a river, then there will be A. one divide wall and one undersluice B. one divide wall and two undersluices C. two divide walls and one undersluice D. two divide walls and two undersluices

The correct answer is $\boxed{\text{C}}$.

A divide wall is a structure that separates two canals. An undersluice is a structure that allows water to flow under a divide wall.

If there are two canals taking off from each flank of a river, then there will be two divide walls and one undersluice. The divide walls will separate the canals from each other, and the undersluice will allow water to flow from the river to the canals.

Option A is incorrect because it only mentions one divide wall. Option B is incorrect because it only mentions one undersluice. Option D is incorrect because it mentions two divide walls and two undersluices, which is not necessary if there are only two canals.