The term Agile Development was first introduced in:

The term "Agile" in the context of software development was formally introduced and defined in the "Manifesto for Agile Software Development," commonly known as the Agile Manifesto. It was created in February 2001 by a group of seventeen software developers who met to discuss lightweight development methods.

A. An addendum called Project Management Principles: This is a generic and incorrect term. The term originated from a specific, well-documented event and document.
C. Lean Process Improvement: While Lean principles from manufacturing heavily influenced many Agile methods, the term "Agile Development" itself was not introduced by the Lean movement. Lean predates the Agile Manifesto.
D. The Scrum Framework: The Scrum Framework was developed in the early 1990s, well before the Agile Manifesto was written in 2001. Scrum is now considered an Agile framework, but it is not the origin of the term.

1. Beck, K., Beedle, M., van Bennekum, A., et al. (2001). Manifesto for Agile Software Development. agilemanifesto.org. (The document itself is the primary source for the term's origin).

2. Highsmith, J. (2001). History: The Agile Manifesto. agilemanifesto.org/history.html. (This page provides the historical context of the meeting where the term was coined and the Manifesto was written).

3. Fowler, M. (2001). The New Methodology. martinfowler.com. (An article by one of the Manifesto's authors describing the meeting and the creation of the term "Agile").