The correct answer is: A. At right angle to the axis of weir.
A divide wall is a structure that is used to divide the flow of water in a channel. It is typically made of concrete or masonry and is placed at right angles to the axis of the channel. The purpose of a divide wall is to prevent the flow of water from one side of the channel to the other. This can be useful for a variety of purposes, such as preventing flooding or ensuring that water is evenly distributed across a field.
The other options are incorrect because they do not describe the correct placement of a divide wall. Option B would not prevent the flow of water from one side of the channel to the other. Option C would actually cause the flow of water to be concentrated on one side of the channel, which could lead to flooding. Option D would not be effective in preventing the flow of water, as the divide wall would not be perpendicular to the axis of the channel.
In conclusion, the correct answer is: A. At right angle to the axis of weir.