The business case is the foundational document that provides the justification for undertaking a project or acquisition. Its primary purpose is to articulate the expected value to the organization by detailing the anticipated benefits (both tangible and intangible), comparing them against the estimated costs, and aligning the initiative with strategic business objectives. For an IS auditor evaluating the software acquisition process, the business case is the most direct and comprehensive source for understanding the intended benefits that drove the investment decision. It serves as the baseline against which post-implementation benefits realization can be audited.

A. A feasibility study primarily assesses the viability of a project, focusing on technical, economic, and operational achievability, rather than detailing the full scope of business benefits.
C. A request for proposal (RFP) outlines functional and technical requirements for vendors; it does not contain the internal justification or the expected benefits for the organization.
D. Alignment with IT strategy is a critical consideration and a component of the business case, but it is not a standalone document that details the specific benefits themselves. ---

1. ISACA, CISA Review Manual, 27th Edition, Domain 2: Information Systems Acquisition, Development, and Implementation, Section 2.3 Project Management. The manual specifies that the business case is the key document that "provides the information required for an organization to decide whether a project should proceed," explicitly including the "expected benefits" as a core component.

2. ISACA, COBIT 2019 Framework: Governance and Management Objectives, Management Objective BAI01: Managed Programs. This framework states that programs are initiated and justified based on a business case, which defines the expected outcomes and benefits, forming the basis for investment decisions and benefits realization tracking.

3. Ward, J., & Daniel, E. (2006). Benefits Management: Delivering Value from IS & IT Investments. John Wiley & Sons. Chapter 4, "Building the Business Case," details how the business case is the formal vehicle for identifying, quantifying, and communicating the expected benefits of an IT investment to stakeholders.