Which of the following alkali metals has the least melting point?

$A$ substance $X$ is a compound of an element of group $IA$. The substance $X$ gives a violet colour in flame test. $X$ is

The oxide of $Cs$ is:

The density of alkali metals is in the order:

The electronic configuration of $Cs$ is

Metals generally melt at very high temperatures. Amongst the following,the metal with the highest melting point will be $......$