Which of the following definitions best describes the term "vapor pressure?a. Pressure and temperature values on a phase diagram where two phases of a substance coexist. b. In equilibrium with the liquid phase, the pressure exerted by a gas. c. Aspecific temperature and pressure at which the liquid and gas phases of a substance have the same density and are indistinguishable from each other. d. The temperature and pressure at which all three phases of a substance coexist. Under these conditions, freezing and melting, boiling and liquefaction, and sublimation and deposition all proceed at the same rate. e. The temperature at which the vapor pressure of a liquid equals 1 atm.

b. In equilibrium with the liquid phase, the pressure exerted by a gas.

Explanation:

When a liquid is warmed up to a temperature , it starts vaporising . The liquid is turning into gas and gas is turning into liquid at different rates  . Initially the rate of former is higher but gradually the difference of rate between them decreases to zero . At this point the rate of conversion of liquid into gas and rate of conversion of gas into liquid becomes equal  . This is called dynamic equilibrium point .

If we change the temperature , the equilibrium gets disturbed .

At this point the pressure exerted by the gas is called the vapour pressure of the liquid .

So option b ) is correct .

a is the correct answer