The correct answer is: A. Always at the ground level.
F.S.L. stands for Full Supply Level. It is the maximum water level that can be stored in a canal without causing any damage to the canal or its surroundings. The F.S.L. is always at the ground level because this is the level at which the canal is designed to operate. If the F.S.L. were above the ground level, the water would flow out of the canal and cause damage. If the F.S.L. were below the ground level, the canal would not be able to hold enough water to meet the needs of the users.
Option B is incorrect because the F.S.L. is always at the ground level. Option C is incorrect because the F.S.L. is not generally 4 to 5 meters above the ground level. Option D is incorrect because the F.S.L. is not only a few cm above the ground level.