Correct Answer - Option 2 : Increase its effectiveness
The symphonic action means to create a negative pressure at lower end so that suction can happen and but if any air is present inside syphon pipe then the pressure inside the pipe cannot be less than the atmospheric and therefore, we need a device that can extract all the air present inside the pipe so that a complete vacuum can be created inside the syphon pipe so that symphonic action could take place.
An overflow spillway is the part of gravity dam. A reinforced concrete hood is constructed over an ordinary overflow section of a gravity dam. The inlet is kept submerged to prevent the entry of debris, ice, etc. This increases the effectiveness of overflow spillway.