ANS:B - compressed air

The air vessel of a reciprocating pump is initially filled with atmospheric air. Here's why: An air vessel is used in reciprocating pumps to smooth out the pulsating flow and reduce pressure fluctuations. When the pump is initially installed and the air vessel is fitted, it is open to the atmosphere, meaning it is filled with atmospheric air. As the pump starts operating, the air in the vessel gets compressed and acts as a cushion to absorb the energy fluctuations caused by the reciprocating action of the pump, leading to a more steady flow of the fluid. Therefore, the initial filling of an air vessel in a reciprocating pump is with atmospheric air.