Answer:
The answer is your first option: Session termination.
Step-by-step explanation:
Basically, a session termination terminates or ends everything that has to do with the user(s). So in this line, "orderly close" means to "neatly or in an organized manner" end. So rather than just pressing the red "X" button on the right, you need to properly doing by ending the session and clicking on the appropriate button.
A bit is a unit of information, so it doesn't have anything to do with "closing a dialogue". A frame overhead is the unit of transmission in a link, so again, nothing to do with what's mentioned above. Packet encapsulation is a method of designing protocols for modular communication... nothing to do as well. And well, finally a message encryption is the process of encoding information, so as to only those authorized can have access to it.