The correct answer is B. Cause and effect diagram . In the CSSBB body of knowledge, gap analysis identifies the difference between current and desired performance, but a team then needs a root-cause tool to determine why that gap exists. The cause and effect diagram , also called a fishbone or Ishikawa diagram, is specifically used to organize and explore potential causes of a problem so the team can identify likely source factors for further verification. The CSSBB material explains that teams brainstorm categories and possible causes in order to investigate root causes systematically. This makes it the most appropriate tool for identifying the source of a performance gap.

The other tools serve different purposes. A matrix diagram shows relationships among sets of items. An activity network diagram is used for scheduling and sequencing project activities. An affinity diagram groups ideas into natural clusters after brainstorming. None of those is primarily intended to identify root causes behind a gap in performance. Since the question asks for the tool to identify the source of the gap, the best CSSBB answer is the cause and effect diagram , which is designed for structured root-cause analysis.