Answer:D. Courteous close
Explanation:courteous close is used when someone is ending some form of an explanation or letter but it is a polite way to show that you are considerate , polite and respectful of others. In this case "Let me know if there is anything else I can do to assist you" it could be said after welcoming someone in their new job , just to show that you consider them and you are available for them if they need you but it is a sign of respect and being considerate.