The hydroxide of which of the following metals reacts with both acid and alkali?

Reaction of $BeO$ with ammonia and hydrogen fluoride gives '$A$' which on thermal decomposition gives $BeF_2$ and $NH_4F$. What is '$A$'?

$A$ solid compound $X$ on heating gives $CO_2$ gas and a residue. The residue mixed with water forms compound $Y$. On passing excess $CO_2$ gas through $Y$,a clear solution $Z$ is formed. On heating $Z$,compound $X$ is recovered. What is compound $X$?
$\mathop X\limits_{solid}$ $\xrightarrow{\text{heating}} CO_2 + \text{residue}$ $\xrightarrow{H_2O} Y$ $\xrightarrow{\text{excess } CO_2} \mathop Z\limits_{\text{solution}}$ $\xrightarrow{\text{heating}} X$

Which one of the alkaline earth metal carbonates is thermally the most stable?
$MgCO_{3}, CaCO_{3}, SrCO_{3}, BaCO_{3}$

Which ion has the maximum complex-forming tendency?

$BeO$ reacts with $HF$ in the presence of ammonia to give $[A]$,which on thermal decomposition produces $[B]$ and ammonium fluoride. The oxidation state of $Be$ in $[A]$ is $.....$.