Explanation
A post-implementation review (PIR) is an assessment conducted at the end of a project cycle to determine if the project was indeed successful and to identify any existing flaws in the project1. One of the main objectives of a PIR is to evaluate the outcome and functional value of a project1. Therefore, an IS auditor should be most concerned with whether the system meets the intended requirements and delivers the expected benefits to the stakeholders. A system that does not have a maintenance plan is a major risk, as it may not be able to cope with changing needs, fix errors, or prevent security breaches. A maintenance plan is essential for ensuring the system's reliability, availability, and performance in the long term2.
The other options are less critical for a PIR, as they are more related to the project management aspects than the system quality aspects. The system may contain several minor defects that do not affect its functionality or usability, and these can be resolved in future updates. The system deployment may be delayed by three weeks due to unforeseen circumstances or dependencies, but this does not necessarily mean that the system is faulty or ineffective. The system may be over budget by 15% due to various factors such as scope creep, resource constraints, or market fluctuations, but this does not imply that the system is not valuable or beneficial.
References: 1: Post-Implementation Review Best Practices - MetaPM 2: What is Post-Implementation Review in Project Management?