Final answer:
A fog-stream is best described as a fine spray composed of tiny water droplets, similar to natural fog or clouds, and is used effectively in firefighting to extinguish fires by absorbing heat and reducing oxygen.
Step-by-step explanation:
The fog-stream that the question likely refers to is most similar to a fine spray composed of tiny water droplets. In firefighting, a fog-stream is used to extinguish fires by creating a water mist, which can absorb heat quickly and reduce the oxygen available to the fire. This dispersion of tiny water droplets resembles fog or clouds in nature, which are also liquid water droplets dispersed in air. In contrast, options such as a semi-solid stream or a compact stream with little shower or spray do not appropriately describe a fog-stream as these terms suggest a different physical form or method of application. Fires can be effectively fought with a fog-stream due to its large surface area when the small droplets vaporize and cool the surrounding area rapidly, making it a highly effective firefighting technique.