The maximum depth of sedimentation tanks, is kept

Answer: Option A

In wastewater engineering, the depth of continuous tanks, particularly sedimentation tanks, is designed within a specified range. The given range for the depth is from 3 to 4.5 meters. When the question specifically asks for the maximum depth, the upper limit of this range is considered. Therefore, the correct choice is Option D: 4.5 m, as it represents the maximum depth within the specified range for continuous tanks in wastewater treatment.