ANS:B - volume of air in cylinder before compression to that after compression.

The compression ratio of an internal combustion (I.C.) engine is the ratio of the volume of air in the cylinder before compression to that after compression. In other words, it's the ratio of the maximum volume (when the piston is at its bottom dead center) to the minimum volume (when the piston is at its top dead center) inside the combustion chamber. The compression ratio is a crucial parameter in determining the efficiency and performance of an engine. A higher compression ratio generally leads to better thermal efficiency and power output, as it increases the pressure and temperature of the air-fuel mixture before ignition, resulting in more efficient combustion.