Answer:
B. Mr. Trelawney can fix it.
Step-by-step explanation:
Dr. Livesey asks Captain Smollett to give Mr. Trelawney his gun in chapters 16-18 of Treasure Island because Mr. Trelawney is an expert with firearms and can fix it. The gun had been damaged in a previous encounter and Mr. Trelawney had offered to repair it. Dr. Livesey believes that Mr. Trelawney is the best person to fix the gun and that it will be more useful to the crew if it is in working order.