Which of the following is considered an informal document?

An email is considered an informal document primarily because of its casual tone and style, which often contrasts with the more structured and precise language found in formal documents. Emails can be used for a variety of purposes, like quick communication, updates, or informal correspondence, which makes them versatile and approachable.

In a business context, informal documents are typically shorter, less structured, and allow for a more personal touch, often reflecting the writer's personality. They facilitate quick exchanges of information and are not held to the same level of formality as formal reports or proposals. Formal reports and requests for proposals (RFPs) are designed to convey detailed information and to adhere to strict formatting and content guidelines, while planning documents often serve strategic purposes and require formality and clarity to ensure all stakeholders understand their roles and responsibilities.