Final answer:
The most common physical access control is A. Lock and key, due to its simplicity and cost-effectiveness, despite other methods like slide cards and biometrics.
Step-by-step explanation:
The most common form of physical access control is A. Lock and key. This traditional method of securing a space involves a mechanical lock and a physical key which must be used to gain access. Other forms of access control include slide cards, biometrics, PIN codes, and alarms. However, lock and key remain the most universally used due to its simplicity and cost-effectiveness. It's important for security purposes to control who has access to different areas and lock and key systems are easy to implement and manage in this context.