What term describes actions that are secretive and often illicit?

The term that describes actions that are secretive and often illicit is "clandestine." This word carries connotations of secrecy, hidden motives, and activities that are typically carried out in a concealed manner, often due to their illicit nature. For example, clandestine meetings may be held to discuss sensitive or illegal activities without drawing attention.

In contrast, the other terms relate to openness and visibility. "Open" and "public" both imply actions or behaviors that are visible and accessible to everyone, which is the opposite of secrecy. "Transparent" suggests a level of openness and clarity in actions or processes, again not aligning with the notion of secrecy or illicit behavior. Thus, "clandestine" is the most appropriate choice to describe those secretive and often unlawful actions.