Final answer:
Mr. Frank emphasized the need for absolute silence, including not flushing toilets, to prevent discovery and ensure the safety of the group hiding from authorities.
Step-by-step explanation:
Mr. Frank stressed to the group that there could be no noise during the day, including not being able to flush the toilet, because it would attract unwanted attention and risk their lives. If they broke this rule, they could potentially alert the authorities or other individuals to their presence, leading to their discovery and the possibility of being captured or killed during a time of heightened surveillance and persecution. This is best reflected in option A, as it directly relates to the need for secrecy and the dire consequences of being discovered.