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

Which among the following elements develops noble gas configuration in $+1$ state?

The metal that is extracted from sea water is

Which of the following metals reacts directly with nitrogen to form a nitride?

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.
$(c)$ The standard electrode potential $E^{\circ}$ for the reaction $M^{2+}_{(aq)} + 2e^{-} \to M_{(s)}$ (where $M = Ca, Sr$ or $Ba$) is nearly constant.

Which element from the following is used in photoelectric cells?