The correct option is C in
In the given question, the preposition ‘above’ is unsuitable here since we are discussing the overflow of water. ‘Above’ is used when we speak about one object being higher than the other. Since in the given context we are speaking about the excess of water and its effect, the concept of one being higher than the other doesn’t exist. Hence option A is incorrect. The preposition ‘at’ is used while speaking about a specific spot or place, but this is not the case in the given sentence. Therefore option B is incorrect. The river and the flood that drowns it are both large bodies and cover a large area. ‘In’ is used to speak about a space or place that is within a larger area, while ‘on’ is used to speak about things that are placed on a flat surface. In the given sentence the flood denotes an excess flow of water that drowns a large body of water which is the river. Hence ‘in’ is suitable since it highlights that the water is not simply on top of the river, rather there is a complete mixing of one another which leads to a flood. Therefore option C is correct, whereas option D is incorrect.