The correct answer is option 3: Dew point. The dew point is the term used to denote the critical temperature at which the air becomes saturated with vapor and below which condensation is likely to begin. When the temperature of the air drops below the dew point, it cannot hold all of the water vapor it contains, resulting in the water vapor condensing into liquid droplets.

Option 1, condensation point, is incorrect because while it refers to the point at which condensation begins, it is not specifically related to the saturation of air with vapor.

Option 2, evaporation point, is incorrect because it refers to the temperature at which a liquid vaporizes, rather than the point at which condensation occurs in the air.

Option 4, point of critical temperature, is incorrect because it is a term unrelated to the saturation of air with vapor.

Therefore, the correct term to denote the critical temperature at which the air becomes saturated with vapor and below which condensation is likely to begin is the dew point.