Which of the following should an IS auditor be MOST concerned with during a post- implementation review?

A robust maintenance plan ensures the system remains secure, performs optimally, and continues to meet business needs over its lifecycle. The absence of such a plan indicates a significant control weakness that could lead to system degradation, security vulnerabilities, or obsolescence, which is a major concern for an IS auditor focusing on ongoing operational controls and risk.

Several minor defects, while undesirable, are common and can be addressed through standard maintenance; their presence is less critical than the lack of a plan for ongoing system health.

A deployment delay is a project management issue that occurred in the past; while it might point to process inefficiencies, it is less of an ongoing control concern for the auditor than the lack of a maintenance plan.

Being over budget is also a past project management issue; while it impacts financial performance, it doesn't directly compromise the system's operational control environment or long-term sustainability as much as the absence of a maintenance plan.