A root cause of a nonconformance is defined as a problem that

The definition of a root cause is the most fundamental reason for the occurrence of a problem or nonconformance. The primary objective of root cause analysis is to identify this underlying issue so that effective corrective action can be taken. By addressing and eliminating the root cause, an organization ensures that the same problem will not happen again. The other options describe tools or processes associated with root cause analysis or nonconformance management, but they do not define the term itself. The core concept is the prevention of recurrence.

A. Designed experiments are a powerful statistical tool used to identify and confirm root causes, but they are not part of the definition.
B. A fishbone (Ishikawa) diagram is a brainstorming tool used to explore and categorize potential causes, not the definition of the actual root cause.
C. The Material Review Board (MRB) is a functional group that decides on the disposition of nonconforming material; its review is a process step, not a definitional characteristic of a root cause.

1. ISO 9000:2015, Quality management systems — Fundamentals and vocabulary.

Section 3.12.2, Note 1 to entry: In the definition of "corrective action" ("action to eliminate the cause of a nonconformity and to prevent recurrence"), the standard distinguishes it from "correction" ("action to eliminate a detected nonconformity"). This emphasizes that addressing the cause is directly linked to preventing recurrence, which is the essence of option D.

2. Russell, J. P. (Ed.). (2021). The ASQ Certified Quality Auditor Handbook (5th ed.). ASQ Quality Press.

Chapter 18, Corrective Action, p. 287: "The root cause is the factor that, when eliminated, prevents the recurrence of the problem... The objective of the root cause analysis is to find the basic cause of the problem so that it can be eliminated." This directly supports that correction must prevent recurrence.

3. Tague, N. R. (2005). The Quality Toolbox (2nd ed.). ASQ Quality Press.

Chapter on Root Cause Analysis, p. 445: "Root cause analysis is an analytical tool used to find the fundamental cause of a problem... The root cause is the cause that, if corrected, will prevent the problem from recurring." This definition explicitly aligns with option D.

4. Purdue University. (n.d.). Root Cause Analysis. Purdue Online Writing Lab (OWL).

In its description of the methodology, the resource states: "The root cause is the core issue—the highest-level cause—that sets in motion the entire cause-and-effect reaction that ultimately leads to the problem(s)." The entire purpose of the analysis described is to find this core issue to implement a permanent fix.