Which of the following stakeholders ensures the business case and program plan are realistic and...

In COBIT 2019 Implementation guidance:

"The Chief Information Officer (CIO) holds responsibility for ensuring the business case is aligned with enterprise objectives and that the program plan is both realistic and achievable, factoring in available resources and capabilities."

The CIO plays a strategic leadership role and has the oversight to balance technology, business needs, risks, and resources. Business process owners and implementation teams contribute, but they do not hold the final accountability for overall feasibility and alignment.