Final answer:
The reaction CO2 + H2O → H2CO3 is a synthesis reaction, where two molecules combine to form a more complex molecule, and does not involve the criteria of combustion, displacement, or decomposition reactions.
Step-by-step explanation:
The reaction CO2 + H2O → H2CO3 is classified as a synthesis reaction. In this type of reaction, two or more simple substances combine to form a more complex substance. This reaction does not fit the criteria for a combustion reaction because it does not involve a substance reacting with oxygen gas to release energy. It is also not a double displacement or single displacement reaction, as those involve an exchange of ions or elements between two compounds. Furthermore, it is not a decomposition reaction, which would involve a single compound breaking down into simpler substances.