A cause that contributes to excessive variation and is detectable using a control chart is...

The correct answer is D. assignable . The CSSBB SPC material explains that process variation can be classified into chance cause variation and assignable cause variation . It further states that when assignable cause variation occurs, SPC facilitates identification of the source so it can be eliminated.

This is the exact idea asked in the question. A control chart is used to detect unusual, non-random variation that indicates the process has changed. Those signals point to assignable causes , also commonly called special causes, such as equipment malfunction, raw material changes, incorrect settings, or operator errors. Common causes are the normal, inherent background variation of a stable process and are not individually identifiable from isolated control-chart signals in the same way. “Rational” relates to rational subgrouping, and “systematic” is not the standard CSSBB term for this purpose. Therefore, the type of cause that contributes to excessive variation and can be detected with a control chart is assignable cause variation . That makes D the verified answer.