Which compound has the lowest thermal stability?

An element $A$ of group $1$ shows similarity to an element $B$ belonging to group $2$. If $A$ has maximum hydration enthalpy in group $1$,then $B$ is:

Choose the correct statement from the following.

Discuss the industrial production of sodium hydroxide ($NaOH$ - Caustic Soda) and mention its properties and uses.

What is the correct order of increasing basic strength for $MgO, SrO, K_2O$,and $NiO$?

$2Na_2O \overset{400 \ ^oC}{\rightleftharpoons} A + 2Na$
$A + SO_2 \to Na_2SO_4$
$A + CO \to Na_2CO_3$
In the above reaction,$A$ is: