What type of reaction occurs when an alkene is treated with ozone (O3)?

When an alkene undergoes treatment with ozone (O3), the reaction that takes place is known as ozonolysis. In this process, the double bond of the alkene reacts with ozone, leading to the formation of a cyclic ozonide intermediate. This intermediate is typically unstable and is further decomposed, often by reduction, to yield carbonyl compounds such as aldehydes or ketones.

Ozonolysis is particularly valuable in organic synthesis because it allows chemists to cleave alkenes in a controlled manner, providing insights into the structure of the starting material and facilitating the synthesis of various functional groups. This reaction is distinct from other processes like hydrogenation, which involves the addition of hydrogen across a double bond, polymerization, a reaction leading to the formation of polymers by linking monomers, and hydrolysis, where water is used to break chemical bonds.

Thus, the specific reaction that occurs when alkene is treated with ozone is ozonolysis, making this the correct answer.