Question

Why is Macbeth upset when he sees Malcolm's army carrying branches from Birnam wood?
a. Because the trees in Birnam wood helped protect the castle
b. Because his wife will be sad that Birnam wood has been destroyed
c. Because he fears the army will set his castle on fire
d. Because one of the witches' prophecies has come true

Answer 1

"d. Because one of the witches' prophecies has come true" would be the best option from the list, since it is the witch's ideas and warnings that are often most feared in this tale. 

Answer 2

The correct answer is D. because one of the witches' prophecies has come true.

At the beginning of the play, a witch made a prophecy about Macbeth - she told him that he will become king soon, but that he will die when the Birnam Wood comes near his castle. He scoffed at this because he thought this was impossible, but when that actually happened, he started fearing for his life (and he did die in the end).