Which of the following is an oxidation-reduction reaction?
so2(g) + h2o(l) h2so3(aq)
caco3(s) cao(s) + co2(g)
ca(oh)2(s) + h2co3(l) caco3(aq) + 2h2o(l)
c6h12o6(s) + 6o2(g) 6co2(g) + 6h2o(l)

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. Reduction reaction is defined as the reaction in which a substance gains electrons.

For the given options:

Option A:

This is a combination reaction because two compounds are reacting to form a single compound.

Option B:

The above reaction is decomposition reaction because a single compound is breaking down into two compounds.

Option C:

The above reaction is a neutralization reaction because an acid reacts with base to produce salt and water.

Option D:

The oxidation state of oxygen changes from 0 to -2, it is gaining electrons. Thus, it is getting reduced and it undergoes reduction reaction. The carbon is getting oxidized and thus it undergoes oxidation reaction.

Thus, it is considered as redox reaction.

Hence, the correct answer is Option D.