Which technology is commonly associated with indoor security applications?

The correct answer highlights the use of indoor microwave technology, which is often employed in security systems for motion detection within buildings. Indoor microwave sensors work by emitting microwave signals that bounce off objects and return to the sensor. When these signals are interrupted by the movement of a person or object, the sensor activates an alarm or notification system. This type of technology is favored for its ability to cover large areas with a relatively low number of sensors, making it efficient for monitoring the safety of indoor environments.

While other options may have roles in security, they are either not as common or not specifically designed for indoor applications. For instance, electric radios are generally not utilized directly in security systems, and photo beams, while effective for certain perimeter detections, are more suited for outdoor use, tracking movement across a line of sight. Active sensors can be part of indoor security systems, but microwave technology provides specific advantages that make it a more common choice for indoor applications.