Final answer:
Cabin crew (CC) are required to secure the flight deck during boarding and at any instances dictated by airline security protocols. The exact times can vary based on specific airline protocols and regulations.
Step-by-step explanation:
The question 'At which level must CC lockdown Flight Deck?' refers to the security protocols that cabin crew (CC) must follow regarding access to the flight deck during a flight. The flight deck must be secured and access restricted during critical phases of flight, which typically includes boarding, turbulence, and other specific situations where security protocols dictate. However, there is no universally applicable answer without specific airline protocols or regulatory guidelines. The most accurate response would be that the flight deck should be locked down according to the airline's procedures, which usually includes during boarding and any time the threat level is elevated.