D core principle of intelligence-led testing frameworks like CBEST is realism. Staging or test environments frequently diverge from production in patch levels, configuration, monitoring coverage, and data - meaning results obtained there would not reliably predict how the organisation's actual defences would perform against a genuine attacker. Testing live production systems (under carefully managed risk controls) is therefore essential to produce a credible assessment of real-world resilience. Cost (D) is not the rationale, staging environments do commonly exist in financial firms (C), and regulators care a great deal about environment choice precisely because it affects the credibility of results (A).