Carlo quit smoking one year ago, but at a party last night he accepted and smoked a cigarette offered to him. in which stage of behavior change is he?

He is at the relapse stage of behavior change,meaning that the patient would again pick up the unhealthy behavior when they had already quit.

In this case,Carlo has already quit smoking,but accepted the cigarette in a party,indicating that he had picked up smoking again and is in the stage of relapse.

Hope it helps!