What type of control uses a radio monitor mode when the controlling agency loses radar capability?

The correct answer is Advisory control. In aviation, when a controlling agency loses radar capability, it often resorts to advising pilots based on the available information. During this radio monitor mode, air traffic controllers may give guidance that helps pilots maintain safe separation from other aircraft and navigate effectively even without radar data. The purpose of advisory control is to keep communication open, allowing controllers to provide situational awareness to pilots, reinforcing safety in the absence of direct radar surveillance.

This option reflects a situation where the emphasis is on communication and guidance rather than direct instruction or directive control, which typically requires radar or more precise tracking capabilities. In contrast, directive control would necessitate a level of oversight that isn't feasible without radar. Broadcast and informative controls focus on sharing information generically with multiple users without a specific interaction needed, and thus they are not the best fit for situations requiring detailed assessment and recommendations that advisory control offers.