Alkali metals form saline hydrides by direct synthesis at high temperatures. Which of the following represents the correct order of decreasing thermal stability of these hydrides?

The number of alkali metal$(s)$,from $Li$,$K$,$Cs$,$Rb$ having ionization enthalpy greater than $400 \ kJ \ mol^{-1}$ and forming stable super oxide is $.......$

What is the order of ionic conductivity for the given cations at a specific concentration?

The correct statements are:
$I$. The low solubility of $LiF$ is due to its high lattice enthalpy.
$II$. $LiBr$ is soluble in acetone.
$III$. $LiCl$ is soluble in pyridine.
$IV$. The order of melting points of alkali metal halides is $MF > MCl > MBr > MI$.

The blue color of a solution of sodium in liquid ammonia is due to:

Given below are two statements:
Statement $I$ : The $4f$ and $5f$-series of elements are placed separately in the Periodic table to preserve the principle of classification.
Statement $II$ : $S$-block elements can be found in pure form in nature.
In the light of the above statements,choose the most appropriate answer from the options given below :