Q1: Fluids change the volume under external presssure due to

ANS:C - compressibility

Fluids change their volume under external pressure due to compressibility. Compressibility refers to the ability of a substance to decrease in volume when subjected to an increase in pressure. In the case of fluids, compressibility describes how easily a fluid's volume can be reduced when pressure is applied. Unlike solids, which have definite shapes and volumes, fluids can change their shape and volume in response to external forces. When pressure is applied to a fluid, its molecules can be forced closer together, resulting in a decrease in volume. This property is particularly important in gases, where compressibility is significant. Plasticity refers to the ability of a material to undergo permanent deformation without breaking when subjected to an applied force. It is a property of solids and is not directly related to the volume change of fluids under pressure. Viscosity, on the other hand, refers to a fluid's resistance to flow and its internal friction. While viscosity affects how fluids flow, it does not necessarily relate to changes in volume under pressure. Therefore, the correct answer is compressibility. Fluids change their volume under external pressure due to their compressibility.