Which of the following is a characteristic of passive RFID tags?

Passive RFID tags are designed to function without an internal power source, which sets them apart from active RFID tags that contain their own battery. Instead, passive tags rely on external signals, typically from RFID readers, to become energized and transmit their data. When an RFID reader emits a radio frequency signal, it creates an electromagnetic field that activates the passive tag. This activation allows the tag to send back its stored information.

The correct answer highlights this reliance on external signals, emphasizing the key operational mechanism of passive RFID technology. Unlike active tags, which can work independently due to their internal power source, passive tags only function when they are within the range of an appropriate RFID reader. This characteristic makes passive tags particularly useful in various applications, such as inventory tracking and supply chain management, where they can be read even without a direct power supply.

Other options, like having their own power source or being used primarily in secure environments, do not accurately describe passive RFID tags and instead apply to different types of RFID technologies or use cases. Also, while passive tags can store data, it is not accurate to say they store data permanently; rather, they hold information that can be altered or overwritten depending on the tag's design and purpose.