Maintenance testing is triggered by changes such as bug fixes, enhancements, or environmental changes.

Option A: "Completion of architecture of the bank system" is not a typical scenario for maintenance testing, as it describes a design phase rather than an operational change.

Option B: "Release of the early draft of the low level project design of an IoT application" is again not suitable for maintenance testing, as it refers to the design phase.

Option C: "Defect was found in a pre-released version of the customer service application" is closer but not quite accurate, as maintenance testing focuses on changes mad​ (ISTQB not-for-profit association)​system is released.

Option D: "Delivery of the hot fix to mobile operating system and ensuring that it still works" is the best example as it directly involves testing after a fix has been implemented.

Therefore, the correct answer is D【6†source】.