The correct option is C Volume.
For combustion reactions, we often enclose all reactants in an explosive-proof steel container, called the bomb whose volume does not change during a reaction. The bomb is then submerged in water or other liquid that absorbs the heat of reaction. The heat capacitor of the bomb plus other things is then measured using the same technique as other calorimeters. Such an instrument is called a bomb calorimeter, and its application is called the bomb calorimetryFor combustion reactions, we often enclose all reactants in an explosive-proof steel container, called the bomb whose volume does not change during a reaction. The bomb is then submerged in water or other liquid that absorbs the heat of reaction. The heat capacitor of the bomb plus other things is then measured using the same technique as other calorimeters. Such an instrument is called a bomb calorimeter, and its application is called the bomb calorimetry.
Since volume does not change, a bomb calorimeter measures the heat evolved under constant volume.