According to CCPS guidance on Incident Investigation and Learning , the size and composition of an investigation team must be determined based on the nature and complexity of the incident and the need to ensure an objective, competent, and balanced team . Option A is correct because more complex incidents—especially those involving multiple system failures, significant consequences, or unclear causal chains— require a larger team with diverse technical expertise (e.g., operations, engineering, safety).

Option D is also correct because CCPS emphasizes that investigation teams must be free from bias and include members with different perspectives to avoid premature conclusions or blame-focused outcomes. A balanced team improves the quality of root cause analysis and ensures credible findings.

Option B is incorrect because CCPS does not mandate the inclusion of external consultants or insurance agents in every investigation. While external experts may be useful in complex or specialized cases, they are not always required.

Option C is incorrect because team size should not depend on volunteers , but rather on required competencies and investigation scope. Allowing volunteers to determine team size can lead to inefficiency or lack of needed expertise.

Ultimately, CCPS stresses that investigation teams should be fit-for-purpose , technically capable, and structured to produce reliable, learning-focused outcomes.