According to the PMBOK® Guide, communication within a project is categorized by its level of formality and the direction of the information flow.

Informal Communication: This includes emails, memos, ad hoc conversations, and social media. While email is a written record, it is technically classified as informal written communication in the context of standard project management terminology. It is used for day-to-day coordination and information exchange that does not require the level of legal or contractual weight found in formal documents.

Formal Communication: This is reserved for official project documents such as the Project Charter, Project Management Plan, status reports to stakeholders, and legal contracts.

Choice of Medium: The project manager selects the communication method based on the Communications Management Plan, which identifies the requirements of the team and stakeholders. Email is the most common form of informal written communication used to manage project work efficiently.

Comparison with other options:

A. Formal: Formal communication typically refers to official reports, briefings, or legal documents. While some high-level emails might be considered " formal, " the standard PMI classification for general email use is informal.

C. Horizontal: This describes the direction of communication (between peers at the same level of the organization) rather than the type or formality of the communication itself.

D. Unofficial: While similar to informal, " unofficial " is not a standard term used in the PMBOK® Guide to classify communication types; the guide strictly uses the Formal/Informal and Written/Verbal axes.