The most important thing for the IS auditor to determine in a post-implementation review of a recently purchased system is whether the user requirements were met. User requirements are the specifications and expectations of the users of the system, such as the features, functions, performance, quality, and security of the system. User requirements are usually defined and documented in the early stages of the system acquisition process, such as in the request for proposal (RFP) or the contract. User requirements are also used as the basis for testing and evaluating the system before and after implementation.
Determining whether the user requirements were met can help the IS auditor assess whether the system is fit for purpose and delivers value and benefits to the users and the organization. Determining whether the user requirements were met can also help the IS auditor identify any gaps, issues, or problems with the system that may affect its functionality, usability, or reliability. Determining whether the user requirements were met can also help the IS auditor provide feedback and recommendations for improvement or enhancement of the system.
Stakeholder expectations were identified is not the most important thing for the IS auditor to determine in a post-implementation review of a recently purchased system, but rather a prerequisite or input for it.
Stakeholder expectations are the needs and wants of the various parties who have an interest or influence in the system, such as users, managers, customers, suppliers, regulators, or auditors. Stakeholder expectations are usually identified and analyzed in the initial stages of the system acquisition process, such as in the feasibility study or the business case. Stakeholder expectations are also used as inputs for defining and prioritizing the user requirements.
Vendor product offered a viable solution is not the most important thing for the IS auditor to determine in a post-implementation review of a recently purchased system, but rather an outcome or result of it. Vendor product is the system that is provided by an external supplier or service provider to meet the user requirements. Vendor product offered a viable solution means that the vendor product satisfied or exceeded the user requirements and delivered value and benefits to the users and organization. Vendor product offered a viable solution can be determined by comparing and evaluating the user requirements and the vendor product performance and quality.
Test scenarios reflected operating activities is not the most important thing for the IS auditor to determine in a post-implementation review of a recently purchased system, but rather a factor or criterion for it. Test scenarios are sets of conditions or situations that are used to test and verify whether the system meets the user requirements. Test scenarios reflected operating activities means that test scenarios simulated or replicated real-world scenarios that occur during normal operations of business processes or functions that use or depend on the system. Test scenarios reflected operating activities can help ensure that test results are valid, reliable, and relevant.
References:
Post Implementation Review: How to conduct and its Benefits 1
Post-implementation reviews - Department of Prime Minister and Cabinet 2 How To Conduct A Post Implementation Audit of Your RecentlyInstalled System 3

The planning phase is the stage of the internal audit process where contact is established with the individuals responsible for the business processes in scope for review. The planning phase involves defining the objectives, scope, and criteria of the audit, as well as identifying the key risks and controls related to the audited area. The planning phase also involves communicating with the auditee to obtain relevant information, documents, and data, as well as to schedule interviews, walkthroughs, and meetings. The planning phase aims to ensure that the audit team has a clear understanding of the audited area and its context, and that the audit plan is aligned with the expectations and needs of the auditee and other stakeholders.
The execution phase is the stage of the internal audit process where the audit team performs the audit procedures according to the audit plan. The execution phase involves testing the design and operating effectiveness of the controls, collecting and analyzing evidence, documenting the audit work and results, and identifying any issues or findings. The execution phase aims to provide sufficient and appropriate evidence to support the audit conclusions and recommendations.
The follow-up phase is the stage of the internal audit process where the audit team monitors and verifies the implementation of the corrective actions agreed upon by the auditee in response to the audit findings. The follow-up phase involves reviewing the evidence provided by the auditee, conducting additional tests or interviews if necessary, and evaluating whether the corrective actions have adequately addressed the root causes of the findings. The follow-up phase aims to ensure that the auditee has taken timely and effective actions to improve its processes and controls.
The selection phase is not a standard stage of the internal audit process, but it may refer to the process of selecting which areas or functions to audit based on a risk assessment or an annual audit plan. The selection phase involves evaluating the inherent and residual risks of each potential auditable area, considering the impact, likelihood, and frequency of those risks, as well as other factors such as regulatory requirements, stakeholder expectations, previous audit results, and available resources. The selection phase aims to prioritize and allocate the audit resources to those areas that present the highest risks or opportunities for improvement.
Therefore, option A is the correct answer.
References:
* Stages and phases of internal audit - piranirisk.com
* Step-by-Step Internal Audit Checklist | AuditBoard
* Audit Process | The Office of Internal Audit - University of Oregon

A key performance indicator (KPI) is a quantifiable measure of performance over time for a specific objective1. KPIs help organizations and teams track their progress and achievements towards their strategic goals. To be useful, a KPI must have a target value, which is the desired level of performance or outcome that the organization or team aims to achieve. A target value provides a clear direction and a benchmark for measuring success or failure. Without a target value, a KPI is meaningless, as it does not indicate whether the performance is good or bad, or how far or close the organization or team is from reaching their objective.