The correct answer is C. Business case.
The business case is the most important document for determining whether an IT investment aligns with organizational objectives. It explains why the investment is being made, what business need it addresses, what benefits are expected, what costs and risks are involved, and how the investment supports strategic or operational goals.
ISACA defines a business case as documentation of the rationale for making a business investment, used both to support the decision to proceed and to support management of the investment through its full economic life cycle. ISACA's CISA Exam Content Outline also specifically includes Business Case and Feasibility Analysis under Domain 3 and includes the task of evaluating whether business cases related to information systems meet business objectives.
Option A is not the best answer because a project risk register identifies and tracks risks, but it does not primarily justify business alignment. Option B is not the best answer because the steering committee charter defines oversight authority and responsibilities, but it does not prove that a specific investment supports organizational objectives. Option D is too narrow because ROI focuses on financial return, while alignment with organizational objectives may include regulatory, operational, strategic, risk reduction, customer service, and resilience objectives.
This maps mainly to Information Systems Acquisition, Development and Implementation because project governance, business case review, feasibility analysis, and investment justification are part of acquisition and development governance.
References: ISACA CISA Exam Content Outline, Domain 3; ISACA Interactive Glossary, "Business case."

The waterfall life cycle model of software development is best suited for situations where the project requirements are well understood. The waterfall life cycle model is a sequential and linear approach to software development that consists of several phases, such as planning, analysis, design, implementation, testing, and maintenance. Each phase depends on the completion and approval of the previous phase before proceeding to the next phase. The waterfall life cycle model is best suited for situations where the project requirements are well understood, as it assumes that the requirements are clear, stable, and fixed at the beginning of the project, and do not change significantly throughout the project. The project is subject to time pressures is not a situation where the waterfall life cycle model of software development is best suited, as it may not be flexible or agile enough to accommodate changes or adjustments in the project schedule or timeline. The waterfall life cycle model may involve long delays or dependencies between phases, and may not allow for early feedback or delivery of software products. The project intends to apply an object-oriented design approach is not a situation where the waterfall life cycle model of software development is best suited, as it may not be compatible or effective with the object-oriented design approach. The object-oriented design approach is a technique that models software as a collection of interacting objects that have attributes and behaviors. The object-oriented design approach may require iterative and incremental development methods that allow for dynamic and adaptive changes in software design and functionality. The project will involve the use of new technology is not a situation where the waterfall life cycle model of software development is best suited, as it may not be able to cope with the uncertainty or complexity of new technology. The waterfall life cycle model may not allow for sufficient exploration or experimentation with new technology, and may not be able to handle changes or issues that arise from new technology.

The best evidence that an organization's IT strategy is aligned to its business objectives is that the IT strategy is approved by executive management. This implies that the IT strategy has been reviewed and validated by the senior leaders of the organization, who are responsible for setting and overseeing the business objectives. The IT strategy may be modified in response to organizational change, based on IT operational best practices, or have significant impact on the business strategy, but these are not sufficient indicators of alignment without executive approval. References: CISA Review Manual (Digital Version)1, Chapter 1, Section 1.2.1

Following a breach, the maximum amount of time before customers must be notified that their personal information may have been compromised depends on the industry regulations that apply to the organization.
Different industries and jurisdictions may have different legal and regulatory requirements for breach notification, such as the General Data Protection Regulation (GDPR) in the European Union, the Health Insurance Portability and Accountability Act (HIPAA) in the United States, or the Personal Information Protection and Electronic Documents Act (PIPEDA) in Canada. Industry standards, incident response plans, and information security policies are not as authoritative as industry regulations in determining the breach notification time frame. References: CISA Review Manual (Digital Version), [ISACA Privacy Principles and Program Management Guide]