The correct answer is: A. 4 to 8 hours.
A plain sedimentation tank is a type of water treatment tank that uses gravity to separate suspended solids from water. The detention period is the amount of time that water spends in the tank. The detention period for a plain sedimentation tank is typically 4 to 8 hours. This allows enough time for the suspended solids to settle to the bottom of the tank, where they can be removed.
Option B is incorrect because the detention period for a plain sedimentation tank is typically 4 to 8 hours, not 8 to 16 hours.
Option C is incorrect because the detention period for a plain sedimentation tank is typically 4 to 8 hours, not 16 to 24 hours.
Option D is incorrect because the detention period for a plain sedimentation tank is typically 4 to 8 hours, not 24 to 36 hours.