Which of the following is an oxidation-reduction reaction? Which of the following is an oxidation-reduction reaction?

Pb(C2H3O2)2(aq) + 2 LiBr(aq) → PbBr2(s) + 2 LiC2H3O2(aq)
KI(aq) + AgNO3(aq) → AgI(s) + KNO3(aq)
HBr(aq) + KOH(aq) → KBr(aq) + H2O(l)
Ba(s) + 2 HCl(aq) → BaCl2(aq) + H2(g)
All of the above are oxidation-reduction reactions.

Explanation:

Oxidation-reduction reaction or redox reaction is defined as the reaction in which oxidation and reduction reactions occur simultaneously.

Oxidation reaction is defined as the reaction in which a substance looses its electrons. The oxidation state of the substance increases.

Reduction reaction is defined as the reaction in which a substance gains electrons. The oxidation state of the substance gets reduced.

On reactant side:

Oxidation state of barium = 0

Oxidation state of hydrogen = +1

On product side:

Oxidation state of barium = +2

Oxidation state of hydrogen = 0

Double displacement reactions are not oxidation reduction reactions as the oxidation state do not change.

Thus is a oxidation reduction reaction.