What is the definition of critical pressure in thermodynamics?

Critical pressure in thermodynamics is defined as the pressure at which a substance cannot exist in two distinct phases - liquid and vapor - regardless of the temperature. At this point, the properties of the liquid and vapor phases become indistinguishable, and the substance exists as a supercritical fluid. This transition occurs at the critical point, which is characterized by critical temperature and critical pressure.

Option B correctly identifies this concept by stating that it is the point where no liquid-vapor phase transition occurs. Above this pressure and at higher temperatures, any liquid present cannot boil into gas, and any gas cannot condense into liquid; they coexist as a supercritical fluid where distinct phases are no longer observable.

The other options do not accurately define critical pressure. For instance, the temperature at which a substance boils pertains to boiling point rather than critical pressure, while the notion of maximum pressure a gas can exert or a liquid existing under pressure does not capture the nuances of phase behavior and the significance of the critical state in thermodynamics.