Which of the following refers to the unwanted reaction of metal with its environment leading to corrosion?

The term that accurately describes the unwanted reaction of metal with its environment that leads to corrosion is "corrosion." This process specifically involves the electrochemical deterioration of materials, usually metals, due to reactions with environmental factors such as moisture, oxygen, salts, or other chemicals.

Corrosion can manifest in various forms, including rusting in iron and steel, which occurs when these metals react with oxygen and water. The term highlights the process's specifics, encapsulating both the cause and effect of the reaction between the metal and its surrounding environment.

Other terms listed, such as oxidation, refer generally to the reaction where a substance loses electrons, often leading to corrosion but not exclusively defining it. Degradation and deterioration are broader terms that encompass various forms of material breakdown and loss of integrity, not confined solely to metal reactions. Therefore, "corrosion" is the most fitting term to describe the unwanted interaction leading to the decline of metallic structures due to environmental exposure.