Among the alkali metals listed in the options, the correct answer is option 4 - Caesium. The melting point of an element is the temperature at which it changes from solid to liquid. In this case, we are comparing the melting points of different alkali metals.

Sodium, option 1, has a melting point of approximately 97.8 degrees Celsius. Potassium, option 2, has a melting point of about 63.5 degrees Celsius. Rubidium, option 3, has a melting point of around 39.3 degrees Celsius.

Caesium, option 4, has the lowest melting point among the alkali metals listed. Its melting point is approximately 28.5 degrees Celsius. This means that Caesium melts at a lower temperature compared to the other alkali metals, making it easier to convert from a solid to a liquid state.

It is important to note that the above answer assumes the given options are correct. If there is reason to believe that the answer provided is incorrect, it is advisable to double-check the information and consider an alternative option.