3. He knows that they cannot move the boulder blocking the doorway.
Explanation:
When the Cyclops attacks Odysseus and his men, Odysseus' first impulse is to attack and kill him to ensure the group's survival. However, they were inside the Cyclops' cave that closed the exit with a very heavy stone, impossible to be removed by humans. Only the Cyclops would be able to open that "door" and if Odysseus killed him, he and his crew would be trapped forever.
