How many electrons are needed to reduce $N_2$ to $NH_3$?

Which of the following is a redox reaction?
$(i) \, CdCl_2 + 2 KOH \longrightarrow Cd(OH)_2 + 2 KCl$
$(ii) \, BaCl_2 + K_2SO_4 \longrightarrow BaSO_4 + 2 KCl$
$(iii) \, CaCO_3 \longrightarrow CaO + CO_2$
$(iv) \, 2 Ca + O_2 \longrightarrow 2 CaO$

In the reaction $M^{+x} + MnO_4^- \to MO_3^- + Mn^{+2}$,if $1 \ mol$ of $MnO_4^-$ oxidizes $1.67 \ mol$ of $M^{+x}$ to $MO_3^-$,find the value of $x$.

Observe the following reaction: $I_2 + 10 HNO_3 \rightarrow 2 HIO_3 + 10 NO_2 + 4 H_2O$. The equivalent weight of $HNO_3$ is (molar mass of $HNO_3 = M$).

In alkaline medium,$MnO_4^{-}$ oxidises $I^{-}$ to:

Assign $A, B, C, D$ from the given type of reaction.
$H_3PO_2 + AgNO_3 \longrightarrow Ag + H_3PO_4 + HNO_3$ (Note: The reaction is balanced as $H_3PO_2 + 4AgNO_3 + 2H_2O \longrightarrow 4Ag + H_3PO_4 + 4HNO_3$)