Final answer:
The most common and inexpensive physical access control device is Key locks. They are widely used due to their simplicity and cost-efficiency.
Step-by-step explanation:
The most common and inexpensive form of physical access control device is D. Key locks. Key locks have been widely used for centuries due to their simplicity, reliability, and relative cost-efficiency. Other methods like security guards, fences, and lighting provide security measures, but when it comes to controlling access to individual doors or entry points, key locks are the most basic and prevalent solution. They are easily installed and managed, which contributes to their popularity as a first line of defense against unauthorized entry.