Which of the following items is NOT considered HAZMAT?

Wheels and brakes are not considered hazardous materials (HAZMAT) because they do not pose any significant risk of injury or harm to people, property, or the environment under normal usage. They are essential components of an aircraft's landing gear and are designed to withstand various physical stresses without releasing any hazardous substances.

In contrast, the other options listed involve materials that have been identified as HAZMAT due to their potential dangers. Liquid oxygen is a strong oxidizer that can exacerbate fires and create explosive environments. Rocket engines contain propellants and other components that can be highly volatile or toxic. Class A combustibles include materials like wood, paper, and some plastics that can ignite and cause fires. Understanding the classification of materials is crucial for safety in aviation maintenance and operations.