When liquefied oxygen is heated from 50K to 300K, first its temperature goes on increasing.After some time the conversion of liquid state to gaseous state initiate. This temperature is known as the boiling point. At boiling point, the liquid state and gaseous state are in equilibrium.After this point for some time the heat applied does not affect the temperature of the liquid oxygen. This is known as latent heat of vaporization.After some time again the temperature starts increasing and the liquid state is completely converted into a gaseous state.Hence correct option is A