Comprehensive and Detailed Explanation From Exact Extract:
This question applies theState Transition Testingtechnique.
Let’s evaluateOption Dstep-by-step:
NORMAL MODE → DEGRADED MODE:✅Valid on“FAILURE DETECTED”
DEGRADED MODE → NORMAL MODE:✅Valid on“ALL FAILURES CLEARED”
NORMAL MODE → DIAGNOSTIC MODE:✅Valid on“SELF DIAGNOSTIC INITIATED”
DIAGNOSTIC MODE → DEGRADED MODE:❌Invalid– there isno defined transitionfrom DIAGNOSTIC MODE directly to DEGRADED MODE.
According to the state diagram, DEGRADED MODE can only be entered:
From NORMAL MODE (on FAILURE DETECTED), or
Not from DIAGNOSTIC MODE at all.
This violates the rule:
“State transition testing uses a model where test cases are designed to execute valid and invalid transitions.”
(ISTQB CTFL Syllabus v4.0, Section 4.2.4 – State Transition Testing, Page 40)
Thus, OptionDcontains aninvalid transition, making it the correct answer.