Which of the following statements about charcoal adsorption devices (CAD) is true?

Charcoal adsorption devices (CAD) are indeed classified as a type of passive testing device, which is the accurate statement here. Passive testing devices, such as CAD, function by allowing radon gas to diffuse into a collection medium—in this case, charcoal—without the need for external power or fans. These devices rely solely on the natural movement of the radon gas into the charcoal over time, which makes them useful for measuring the presence of radon in the environment without requiring active intervention or continuous monitoring.

In contrast, other types of testing devices, such as active devices, use fans or pumps to actively draw air through a measurement chamber, often providing more immediate and continuous data on radon levels. Charcoal adsorption devices do not perform in this manner and instead require an exposure period, followed by laboratory analysis to determine the radon concentration based on the amount absorbed by the charcoal.

Thus, labeling CAD as passive devices is grounded in their operational principle and function, making this option the correct choice.