ANS:A - volume

In the context of the Otto cycle, "constant volume" refers to a specific phase of the cycle where the combustion of fuel and air mixture occurs at constant volume within the combustion chamber. During this phase, the piston remains stationary, and the volume within the cylinder remains constant as the fuel-air mixture undergoes combustion. This allows for efficient conversion of chemical energy into mechanical work, which drives the piston down to produce power in the engine.