The correct answer is: A. Ground slope exceeds the designed bed slope.
A fall in a canal bed is generally provided to ensure that the water flows smoothly and does not stagnate. If the ground slope exceeds the designed bed slope, then the water will flow too quickly and may cause erosion. On the other hand, if the designed bed slope exceeds the ground slope, then the water will flow too slowly and may become stagnant. Therefore, the only option that ensures that the water flows smoothly is A.
Option B is incorrect because if the designed bed slope exceeds the ground slope, then the water will flow too slowly and may become stagnant. This can lead to the growth of algae and other aquatic plants, which can clog the canal and make it difficult for water to flow.
Option C is incorrect because if the ground slope is practically the same as the designed bed slope, then there is no need to provide a fall in the canal bed. The water will flow smoothly without any problems.
Option D is incorrect because it is not a valid option.