Answer: C. He knows that they cannot move the boulder blocking the doorway.
The Cyclops had blocked the entrance to his cave and this prevented Odysseus from killing him. Odysseus had drafted out a plan for the stone to be moved by the Cyclops. Without this, they’d be stuck there with no food or water. While still trapped in the cave, Odysseus crafted a spear from a piece of wood in the cave and then gave the Cyclops wine so as to get him drunk. This was the only way to escape, using the one-eyed giant to open the cave for them