(a) Why does a gas exert pressure?(b) Why does a gas fill a vessel completely?(c) Why are gases so easily compressible whereas it is almost impossible to compress a solid or a liquid?

(a) When gas is stored in a container, the fast-moving particles of the gas collide with each other and with the walls of the container. Thus, exerting pressure on the walls of the container.  

(b) As we know, gases have large inter-molecular space and weak intermolecular force of attraction. Due to these properties, gases do not have a fixed shape and volume and can move freely in the vessel. Hence, the gas particles spread out and move away from each other filling the vessel completely.

(c) Gases are easily compressible as they have large inter-molecular spaces. The gas particles can come closer to each other when external pressure is applied to gases.

On the other hand, it is almost impossible to compress a liquid because the intermolecular spaces in liquids are small. The particles in a solid are very close to each other. So, they cannot be compressed by applying external pressure.