The IS auditor’s greatest concern when reviewing a business case for a proposed implementation of a third-party system should be A. Lack of ongoing maintenance costs. This is because ongoing maintenance costs are an essential part of the total cost of ownership (TCO) of a third-party system, and they can have a significant impact on the return on investment (ROI) and the feasibility of the project. If the business case does not include ongoing maintenance costs, it may underestimate the true cost of the project and overestimate the benefits. This could lead to poor decision making and unrealistic expectations.

Lack of training materials (B), lack of plan for pilot implementation ©, and lack of detailed work breakdown structure (D) are also potential issues that could affect the quality and success of the project, but they are not as critical as lack of ongoing maintenance costs. Training materials can be developed or acquired later, pilot implementation can be planned during the project initiation or planning phase, and work breakdown structure can be refined as the project progresses. However, ongoing maintenance costs are difficult to change or estimate once the project is approved and implemented, and they can have long-term implications for the organization. Therefore, they should be included and analyzed in the business case.