The correct answer is B) prevents the sling ring from coming out of the hook. Mousing a cargo hook with marline or small line helps prevent the sling ring from accidentally coming out of the hook during a lift. This is an important safety precaution to ensure the load remains securely attached to the hook. Mousing the hook does not increase its lifting capacity (option C) or directly protect the hook from the sling ring (option A). The primary purpose is to keep the sling ring in place on the hook, which makes option B the correct answer.