Final answer:
Certified mail provides the sender with proof of delivery, making it the correct answer to which type of mail confirms receipt. It is used for legal or important documents when delivery confirmation is necessary.
Step-by-step explanation:
The type of mail that provides proof that a letter has been received is certified mail. When you send a letter via certified mail, the postal service provides you with a tracking number, and you can request a return receipt, which gives you a record that the recipient has indeed received the mail. This service is often used for legal documents, important business correspondence, or any other time when the sender needs proof of delivery.