Which statement best describes a master key system?

• A. A system where a single key opens only one lock
• B. A system with multiple master keys but no single master key
• C. A system that uses digital keys only
• D. System with two or more levels of key operation when there is a single master key that fits all locks

Answer: (D)

Master key systems organize access with multiple levels. Each lock is paired with a change key for its specific door, while a master key opens all locks in the system, and larger setups can add higher levels (like sub-master or grand master) to extend or limit access as needed. The statement that describes this idea best says there are two or more levels of key operation, with a single master key that fits all locks. That captures the core structure: specific locks respond to their change keys, and a single master key can open every lock in the system. The other options describe only one-key-per-lock, or restrict concepts to digital keys, or omit the existence of a single master key that opens everything, which misses the defining feature of a master key system.