Answer:
B. It marks Macbeth's eventual defeat.
In the Birnam Wood scene of Macbeth, the army that is coming to overthrow Macbeth is camouflaging themselves with tree branches from Birnam Wood. This is significant because earlier in the play, Macbeth was told by a prophecy that he would only be defeated when Birnam Wood moved to Dunsinane. The camouflage technique is a way of fulfilling the prophecy, without actually moving the entire forest. This scene thus marks Macbeth's eventual defeat, and highlights the supernatural forces working against him.