Which option best describes a "Security Agency"?

The correct choice accurately defines a "Security Agency" as a person, firm, corporation, or other private legal entity that is specifically engaged in providing security guard services. This encompasses a broad range of activities focused on the protection of property and individuals, encompassing duties such as monitoring premises, responding to emergencies, and preventing criminal activities.

Understanding this definition is crucial for recognizing the various entities involved in the security industry and distinguishing them from those who may not primarily provide security services. Options like a self-employed private investigator or armored car companies do not capture the essence of what constitutes a security agency, which is centered on offering security guard services as a primary function. Similarly, while an individual contracting to prevent theft may perform security-related tasks, they do not classify as a formal agency in the context of the security industry, as they lack the structure of a firm or corporation dedicated wholly to security services.