Which element in group $IA$ (Alkali metals) exhibits the highest chemical reactivity?

The strongest reducing agent among the following is:

Magnesium is burnt in the air to form $A$ and $B$. When $B$ is hydrolyzed,$C$ and $D$ are formed. $D$ is the reactant in the manufacture of nitric acid by Ostwald's process. What is $C$?

Which of the nitrates on strong heating leaves the metal as the residue?

Among the statements $(a)-(d)$ the correct ones are
$(a)$ Lithium has the highest hydration enthalpy among the alkali metals.
$(b)$ Lithium chloride is insoluble in pyridine.
$(c)$ Lithium cannot form ethynide upon its reaction with ethyne.
$(d)$ Both lithium and magnesium react slowly with $H_2O$.

Identify the correct statements from the following:
$A$) Among alkali metal ions,$Li^+$ has the highest hydration enthalpy.
$B$) The boiling point of alkali metals increases from $Li$ to $Cs$.
$C$) The density of $K$ is less than that of $Na$ and $Rb$.
$D$) $Li$ has a strong tendency to form superoxide.