What is significance of malcolm's army carrying branches from birnam wood to the castle

Isaw clearly the doom which had been prepared for me, and congratulated myself upon the timely accident by which ! had escaped. another step before my fall, and the world had seen me no more and the death just avoided was of that very character which i had regarded as fabulous and frivolous in the tales respecting the inquisition. to the victims of its tyranny, there was the choice of death with its direst physical agonies, or death with its most hideous moral horrors. i had been reserved for the latter. by long suffering my nerves had been unstrung, until i trembled at the sound of my own voice, and had become in every respect a fitting subject for the species of torture which awaited me. which torture does the passage above reference? a. being eaten alive by rats b.being sliced open by a giant blade c.being killed by falling into a hole d. being burned alive