Final answer:
Board sags in ceilings can be caused by improperly fitted panels, insufficient support of board, and high-humid conditions, therefore, all of the mentioned factors contribute to ceiling board sagging.
Step-by-step explanation:
Board sags in ceiling can be caused by various factors. These may include improperly fitted panels, which are panels that have not been cut or installed correctly, leading to gaps and eventual sagging. Another common cause of sagging is insufficient support of board; this occurs when the framing or joists meant to hold up the boards are spaced too far apart or are not providing adequate stability. Finally, high-humid conditions can also cause ceiling boards to sag as moisture changes can affect the structural integrity of the board material. Therefore, the correct answer to the question posed is 'D. All of the above.'