Final answer:
Malcolm's army makes the witches' prophecy come true by taking branches from Birnam Wood and using them as camouflage, creating the illusion of the forest moving towards Macbeth's castle as the prophecy predicted.
Step-by-step explanation:
Malcolm's army makes the witches' prophecy come true by adopting a clever strategy to fulfill part of the prophecy related to Birnam Wood. The correct answer is B. The army breaks off branches from the trees in Birnam wood and carries the branches with them as they march on Macbeth's castle. This action makes it appear as if the wood is moving toward Dunsinane Hill, which the witch's prophecy foretold as a sign of Macbeth's downfall.