Which of the following is a common form of communication for security guards?

Two-way radios are a prevalent form of communication for security guards because they enable instant and efficient communication over short to medium distances. This real-time communication is crucial in security settings, where immediate alerts or coordination may be necessary to respond to incidents effectively. Two-way radios are designed to be durable and suitable for the often fast-paced and unpredictable environments that security personnel operate in, allowing them to maintain situational awareness and respond quickly to emergencies.

Other options, such as cell phones, text messaging apps, and email, while they can be used for communication, may not offer the same immediate connectivity and reliability as two-way radios in high-pressure situations. Cell phones may be subject to delays due to network congestion, and while text messaging can be useful, it lacks the immediacy needed in urgent security scenarios. Email is typically not suitable for real-time communication due to its slower response times. Thus, two-way radios stand out as a more effective tool for security guards in maintaining real-time communication while on duty.