Inter-distance between ventilation columns in a sewer line is kept

ANS:D - 150 m to 300 m.

The inter-distance between ventilation columns in a sewer line is typically kept in the range of 150 meters to 300 meters. Ventilation columns are installed along sewer lines to provide ventilation and release foul gases trapped within the sewer system. The spacing between these ventilation columns depends on various factors such as the length of the sewer line, the topography of the area, the flow characteristics of the sewage, and the need to ensure adequate ventilation throughout the system. Therefore, the correct option is: d. 150 m to 300 m.