The correct answer is: A. Drainage passes over the canal and F.S.L. of the canal is below the bottom of the drainage trough.
A syphon aqueduct is a type of aqueduct that uses the principle of siphoning to transport water over an obstacle. In a syphon aqueduct, the water is carried in a pipe that is above the level of the obstacle. The water is able to flow through the pipe because the pressure of the water at the bottom of the pipe is greater than the pressure of the air at the top of the pipe.
The F.S.L. (full supply level) of the canal is the level of the water in the canal when it is full. The bottom of the drainage trough is the lowest point in the drainage trough. In a syphon aqueduct, the F.S.L. of the canal is below the bottom of the drainage trough. This is because the water in the canal is able to flow through the pipe even though the pipe is above the level of the obstacle.
The other options are incorrect because they do not describe the correct way that a syphon aqueduct works. In option B, the F.S.L. of the canal is above the bottom of the drainage trough. This would not allow the water to flow through the pipe because the pressure of the water at the bottom of the pipe would be less than the pressure of the air at the top of the pipe. In option C, the canal passes over the drainage. This would not allow the water to flow through the pipe because the water in the canal would be blocked by the drainage. In option D, the canal passes over the drainage and the H.F.L. (high flood level) of the drainage is below the bottom of the canal trough. This would not allow the water to flow through the pipe because the pressure of the water at the bottom of the pipe would be less than the pressure of the air at the top of the pipe.