Final answer:
Carbon monoxide reacts with hydrogen to form methanol, which is a reduction option (2) reaction as defined by the addition of hydrogen to a molecule in organic chemistry.
Step-by-step explanation:
The reaction of carbon monoxide (CO) with hydrogen (H2) to form methanol (CH3OH) involves the reduction of carbon monoxide. This is because reduction in organic chemistry can be characterized by the addition of hydrogen to a molecule.
When CO reacts with H2, the carbon atom in CO gains hydrogen atoms and transforms into methanol, thereby it undergoes reduction.
In organic chemistry, oxidation can be understood as the addition of oxygen to a substance or the removal of hydrogen. Because oxygen is not being added to CO in this reaction, and hydrogen atoms are added instead, CO undergoes reduction, not oxidation.
This is the opposite of combustion reactions, where a carbon-containing material reacts with oxygen, resulting in the oxidation of the carbon, indicated by the formation of carbon dioxide and water.