Which of the following reactions is classified as a redox reaction?

A redox reaction, short for reduction-oxidation reaction, involves the transfer of electrons between chemical species, leading to changes in their oxidation states. In the reaction of hydrogen and oxygen to form water, one of the reactants (hydrogen) undergoes oxidation, while the other reactant (oxygen) undergoes reduction.

In the given reaction, molecular hydrogen (H₂) is oxidized as it loses electrons to form protons (H⁺), while molecular oxygen (O₂) gains those electrons, thereby reducing its oxidation state. The overall electron transfer indicates a change in the oxidation states of hydrogen and oxygen, marking this as a redox reaction.

The other reactions listed do not involve this kind of electron transfer that results in a change in oxidation states. The formation of ammonia from nitrogen and hydrogen is not redox since neither element undergoes a change in oxidation state; carbon and oxygen combining to form carbon dioxide does not involve a reduction or oxidation of carbon in a significant way when compared to the format of this redox reaction. The neutralization of hydrochloric acid with sodium hydroxide resulting in sodium chloride and water also does not involve electron transfer or changes in oxidation states, thus not qualifying it as a redox reaction.