Which one among the following metals is the weakest reducing agent?

What else is present in a fire extinguisher along with ${H_2}SO_4$?

Assertion $(A)$: Alkali metals and their salts impart characteristic flame colours.
Reason $(R)$: Alkali metals have low ionization enthalpy values. So,electron excitation is possible.
The correct option among the following is

$Na + Al_2O_3$ $\xrightarrow{\text{High temperature}} X$ $\xrightarrow[\text{water}]{CO_2} Y$; compound $Y$ is

Comment on each of the following observations:
$(a)$ The mobilities of the alkali metal ions in aqueous solution are
$Li^{+} < Na^{+} < K^{+} < Rb^{+} < Cs^{+}$
$(b)$ Lithium is the only alkali metal to form a nitride directly.
$(c)$ $E^{\circ}$ for $M^{2+}_{(aq)} + 2e^{-} \longrightarrow M_{(s)}$ (where $M = Ca, Sr$ or $Ba$) is nearly constant.

Which of the following alkali metals is smallest in size?