Which of the following equations represents an oxidation Zreduction reaction?

O.N. = oxidation number

A.
In H₂, the O.N. of H = 0
In H, the O.N. of H = 0
This is NEITHER an oxidation NOR a reduction reaction, because there is no change in O.N.

B.
In N₂O₂, the O.N. of N = +2, and the O.N. of O = -2
In NO₂, the O.N. of N = +4, and the O.N. of O = -2
This is an OXIDATION reaction, because the O.N. of N increases.

C.
In O⁻, the O.N. of O = -1
In O₂⁻, the average O.N. of O = -1/2
This is a REDUCTION reaction, because the O.N. of O decreases.

D.
In H₂, the O.N. of H = 0
In Cl₂, the O.N. of Cl = 0
In HCl, the O.N. of H = +1, and the O.N. of Cl = -1
This is an OXIDATION-REDUCTION reaction, because the O.N. of H increases (oxidation) and that of Cl decreases (reduction).

E.
In HCl, the O.N. of H = +1, and the O.N. of Cl = -1
In NaBr, the O.N. of Na = +1, and the O.N. of Br = -1
In HBr, the O.N. of H = +1, and the O.N. of Br = -1
In NaCl, the O.N. of Na = +1, and the O.N. of Cl = -1
This is NEITHER an oxidation NOR a reduction reaction, because there is no change in O.N.

...... The answer is: D. H₂ + Cl₂ → 2HCl