Which of the following has the highest electrode potential?

The setting of Plaster of Paris involves:

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

Bleaching powder is obtained by treating chlorine with

Which of the following elements has the highest tendency to form a peroxide?

The alkaline earth metal sulphate,which has its hydration enthalpy greater than its lattice energy is