Which maintenance process model focuses on fixing defects as they are…

Concept: Per the ISO/IEC/IEEE 14764 standard, software maintenance is classified into four types by the reason for the change: corrective maintenance reactively fixes defects found after delivery; perfective maintenance improves performance, usability, or functionality; adaptive maintenance adjusts the software to a changed operating environment (OS, hardware, regulations); and preventive maintenance improves maintainability before problems occur.

Application: The question asks specifically for the maintenance type driven by fixing defects once they are identified in a live system — by definition, this is corrective maintenance.

Cross-check — contrasting the other options by what actually drives each:

• Waterfall Model is not a maintenance category at all — it is a sequential software development lifecycle model (requirements, design, implementation, testing, deployment), unrelated to post-release upkeep.

• Perfective Maintenance is driven by new or changed user requirements — performance or feature improvements — not by discovered defects.

• Adaptive Maintenance is driven by changes in the operating environment (new OS, hardware, regulation), not by discovered defects.

Result: Corrective Maintenance is the maintenance type that focuses on fixing defects as they are identified.