Answer:
The correct answer is: "A. switching your cell phone off before you enter a meeting".
Step-by-step explanation:
Letter A is the only correct option because it is more advisable to turn off completely mobiles, watches, or any other kind of technology tool which may cause some noise and interrupt a business meeting and others. All other options are lacking in some way because they do not sound or seem like real good business etiquette for they are incomplete or present the opposite idea of turning tools totally off.