Which of the following is an example of informal written communication?

Informal written communication is characterized by a conversational tone and less rigid structure compared to formal communication methods. A memo regarding a shirt order fits this definition perfectly. Memos are often used for quick, internal communications within an organization, and the topic itself is casual and straightforward.

In contrast, the other options generally imply a more formal context. A letter to a citizen often adheres to formal tone and structure, as it typically involves communication from an organization to the public. Department operating procedures are formal documents outlining rules or regulations within an organization, which serve a specific purpose and require adherence to a strict format. A monthly activity report summarizes official activities over the month and is expected to maintain a level of professionalism and structure. The memo about a shirt order stands out as casual and informal among these options, highlighting its suitability as the example of informal written communication.