The answer would be complex sentence, because it consists of an individual clause (The explorers brightened the dark cave) and a dependent clause (when they lit a torch from their cooking fire).
B. a letter. Macbeth was given his prophecy by his meeting with the witches that he will become king. He sends a letter to communicate his prophetic message to his lady, who encourages him to murder King Duncan when news arrives that he will be visiting Inverness. She manipulates her husband Macbeth, stating he is weak and persuades him to kill the King to hasten the prophecy of becoming king.