The primary reason for an IS auditor to conduct post-implementation reviews is to determine whether project objectives in the business case have been achieved. A post-implementation review is an audit activity that evaluates whether a project has delivered its expected outcomes or benefits in accordance with its objectives, scope, budget, and schedule. A business case is a document that defines and justifies the need, value, and feasibility of a project. A post-implementation review can help assess whether project objectives in the business case have been achieved by comparing actual results with planned expectations and identifying any gaps or deviations. The other options are not primary reasons for conducting post-implementation reviews, as they do not measure whether project objectives in the business case have been achieved. Ensuring key stakeholder sign-off has been obtained is a project closure activity that confirms that all project deliverables have been completed and accepted by key stakeholders, but it does not evaluate whether project objectives in the business case have been achieved. Aligning project objectives with business needs is a project initiation activity that ensures that the project is aligned with the organization's strategy, goals, and priorities, but it does not evaluate whether project objectives in the business case have been achieved. Documenting lessons learned to improve future project delivery is a project learning activity that captures and shares the knowledge, experience, and feedback gained from the project, but it does not evaluate whether project objectives in the business case have been achieved. References: CISA Review Manual (Digital Version), Chapter 3, Section 3.3