The temperature at which the vapour pressure of a liquid becomes equals of the external pressure is its. (A) Melting point (B) sublimation point (C) Critical temperature (D) Boiling point

The statement can be rephrased as follows: "The temperature at which the vapor pressure of a liquid becomes equal to the external (atmospheric) pressure." This means that at this particular temperature, the liquid has enough energy to overcome the external pressure and change its phase from liquid to gas.

Now, let's go through each option and understand their meanings: (A) Melting point: The temperature at which a solid turns into a liquid under standard atmospheric pressure. This option doesn't match our statement since it refers to a solid-to-liquid phase transition. (B) Sublimation point: The temperature at which a substance changes directly from the solid phase to the gas phase without passing through the liquid phase. This option also doesn't match our statement since it doesn't involve a liquid-to-gas phase transition. (C) Critical temperature: The temperature above which a gas cannot be liquefied, regardless of the pressure applied. This option also doesn't match our statement since it doesn't describe the situation when vapor pressure equals the external pressure. (D) Boiling point: The temperature at which a liquid's vapor pressure equals the external (atmospheric) pressure, causing it to vaporize and change from the liquid phase to the gas phase. This option matches the given statement perfectly, as it describes the exact scenario mentioned in the statement.

Based on our analysis, the correct answer is (D) Boiling point, as it accurately describes the temperature at which the vapor pressure of a liquid becomes equal to the external (atmospheric) pressure, causing the liquid to change its phase from liquid to gas.