Final answer:
The most visible element of security is the uniform, which identifies security guards and serves as a visual deterrent.
Step-by-step explanation:
The most visible element of security is the uniform, which is option A. A uniform is a standardized attire worn by security guards that helps identify them as official personnel responsible for maintaining security. The uniform serves as a visual deterrent to potential threats and reassures individuals that security measures are in place.
While options B and C include a security guard, it is the uniform that is specifically mentioned as the most visible element. Option D, frequent supervisory visits, may be important for ensuring the effectiveness of security measures but is not the focus of visibility in this context.