What is the letter called that responds to a challenge letter

Response letter, reply letter. A letter in response to a query letter is often called a "response letter" or "reply letter," and is a letter that responds to a query letter received to answer the query, provide relevant information, or resolve a dispute. A response letter usually includes specific answers, explanations and evidence to the query, as well as a solution or action plan, and is designed to facilitate effective communication and problem solving, and to maintain good communication and cooperative relationships.