Final answer:
In the event of a fire alarm sounding and the facility filling with smoke, the appropriate action is to move clients behind a fire door and close the door.
Step-by-step explanation:
In the event of a fire alarm sounding and the facility filling with smoke, but no visible flames, the appropriate action would be to move clients behind a fire door and close the door (option A).
Moving clients behind a fire door provides a barrier between them and the fire, helping to prevent the spread of smoke and flames. Closing the door further helps to contain the smoke and heat.
By selecting this option, the clients are being protected and the fire is being contained until further instructions or assistance can be requested.