list detected;
list buttons;
default
{
    state_entry()
    {
        llSay(0, "Hello, Avatar!");
    }

    sensor(integer num_detected){
        buttons =[];
        detected=[];
        integer i;
        while(i<num_detected){
            key k = llDetectedKey(i);
            buttons=[llGetSubString(llGetDisplayName(k),0,23)]+buttons;
            detected=[k]+detected;
            i++;
        }

        llDialog(av,"Please choose someone",buttons,chan);

    }

    listen(integer channel, string name, key id, string msg){
        integer n = llListFindList(buttons,[msg]);
        if(~n){// or n!=-1 if you prefer
          key target = llList2Key(detected,n);
        }

    }
}