Which of the following options is NOT a use of illumination?

The option that is identified as not a use of illumination is the correct choice because marking friendly locations for Close Air Support (CAS) attacks does not commonly involve the use of illumination munitions. Instead, the primary purposes of illumination are to enhance visibility and increase situational awareness during nighttime operations. This includes illuminating areas where adversary activity is suspected, providing light for adjusting fires at night, and potentially harassing enemy positions through increased visibility.

Illumination is primarily utilized to reveal enemy positions or movements, thereby allowing friendly forces to react appropriately under the cover of darkness. In contrast, marking a friendly location for CAS is typically achieved through other means, such as using smoke or laser designators, which are better suited for this purpose rather than using illumination rounds, which are intended to light an area rather than indicate a specific friendly location.