Which of the following has the highest electrode potential?

$A$ compound $X$ on heating gives a colourless gas. The residue is dissolved in water to obtain $Y$. Excess $CO_2$ is bubbled through the aqueous solution of $Y$ and $Z$ is formed. $Z$ on gentle heating gives back $X$. The compound $X$ is:

Select the incorrect order of solubility.

Select the incorrect statement about alkaline earth metals.

$Beryllium$ shows a diagonal relationship with:

Alkaline earth metals belong to the