The most appropriate one-word substitute for a formal statement testifying to someone's character and qualifications is "Testimonial."

• Document: While a testimonial can be a type of document, it specifically refers to a statement of character and qualifications, whereas a document can encompass a wide range of written materials.
• Affidavit: An affidavit is a written statement made under oath, typically used as evidence in a legal context. It may include information about a person's character and qualifications, but it is not specifically focused on that aspect.
• Testimonial: This is the correct option. A testimonial is a formal statement that provides an endorsement or affirmation of someone's character, abilities, or qualifications.
• Manuscript: A manuscript refers to a handwritten or typewritten document, typically of a literary or academic nature. While it can contain information about a person's character or qualifications, it does not specifically indicate a formal statement or endorsement.