The best answer is D. Feasibility study.
ISACA guidance notes that, in projects, an auditor reviews the feasibility of projects based on resources, time, and other factors, and evaluates whether projects are aligned with the organization's strategic goals. That is exactly the stage where internal audit's advisory involvement is most appropriate during development of the business case.
Option A, B, and C occur later in the system development lifecycle and are more operational or project- delivery focused. The question is specifically about the development of a business case, and feasibility analysis is the phase most directly tied to whether the initiative is practical, justified, and aligned with business goals.
References (Official ISACA):
* ISACA Journal, Innovation and the Auditor.
* ISACA, An Appropriate Approach for Program and Project Management.

The major advantage of automating internal controls is to efficiently test large volumes of data, because automated controls can perform repetitive tasks faster, more accurately, and more consistently than manual controls. Automated controls can also provide audit trails and exception reports that facilitate the monitoring and evaluationof the control effectiveness12. Reviewing large value transactions, identifying transactions with no segregation of duties, and performing analytical reviews are possible benefits of automating internal controls, but not the major advantage. References: 1: CISA Review Manual (Digital Version), Chapter 5, Section 5.2.2 2: CISA Online Review Course, Module 5, Lesson 2

The best way to evaluate the effectiveness of a new automated control is to review the written procedures that define the processes and controls. This will help the IS auditor to understand the objectives, scope, roles, responsibilities, and expected outcomes of the control. The written procedures will also provide a basis for testing the control and verifying its compliance with the audit finding recommendations. References:
ISACA Frameworks: Blueprints for Success
CISA Review Manual (Digital Version)

The most important responsibility of user departments associated with program changes is approving changes before implementation. This is because user departments are the primary stakeholders and beneficiaries of the program changes, and they need to ensure that the changes meet their requirements, expectations, and objectives. User departments also need to approve the changes before implementation to avoid unauthorized, unnecessary, or erroneous changes that could affect the functionality, performance, or security of the program.
Providing unit test data is a responsibility of user departments associated with program changes, but it is not the most important one. Unit test data is used to verify that the individual components of the program work as expected after the changes. However, unit test data alone cannot guarantee that the program as a whole works correctly, or that the changes are aligned with the user departments' needs.
Analyzing change requests is a responsibility of user departments associated with program changes, but it is not the most important one. Analyzing change requests is the process of evaluating the feasibility, necessity, and impact of the proposed changes. However, analyzing change requests does not ensure that the changes are implemented correctly, or that they are acceptable to the user departments.
Updating documentation to reflect latest changes is a responsibility of user departments associated with program changes, but it is not the most important one. Updating documentation is the process of maintaining accurate and complete records of the program's specifications, features, and functionsafter the changes.
However, updating documentation does not ensure that the changes are effective, or that they are approved by the user departments.
References:
ISACA, CISA Review Manual, 27th Edition, 2019, p. 281
ISACA, CISA Review Questions, Answers and Explanations Database - 12 Month Subscription