Senior management support is essential to ensuring effective incident response because it provides the necessary authority, resources, and guidance for the information security team to perform their roles and responsibilities. Senior management support also helps to establish the goals, scope, policies, and procedures for the incident response plan (IRP), as well as to ensure its alignment with the business objectives and strategy. Senior management support also fosters a culture of security awareness, accountability, and collaboration among all stakeholders involved in the incident response process.
The other options are not essential to ensuring effective incident response, although they may be helpful or beneficial. A business continuity plan (BCP) is a document that outlines the actions and arrangements to ensure the continuity of critical business functions in the event of a disruption or disaster. A cost-benefit analysis is a method of comparing the costs and benefits of different alternatives or solutions to a problem. A classification scheme is a system of categorizing information assets based on their sensitivity, value, and criticality.
References = CISM Manual1, Chapter 6: Incident Response Planning (IRP), Section 6.1: Incident Response Plan2
1: https://store.isaca.org/s/store#/store/browse/cat/a2D4w00000Ac6NNEAZ/tiles 2: 4

The first step when developing a business case for a new intrusion detection system (IDS) solution is to define the issues to be addressed. A business case is a document that provides the rationale and justification for initiating a project or investment. It typically includes information such as the problem statement, the objectives, the alternatives, the costs and benefits, the risks and assumptions, and the expected outcomes. The first step in developing a business case is to define the issues to be addressed, which means identifying and describing the current situation, the problems or challenges faced by the organization, and the needs or opportunities for improvement. By defining the issues to be addressed, the information security manager can establish the scope and purpose of the business case, and provide a clear and compelling problem statement that explains why a new IDS solution is needed. The other options are not the first step when developing a business case for a new IDS solution, although they may be part of the subsequent steps. Performing a cost-benefit analysis is a step that involves comparing the costs and benefits of different alternatives, including the new IDS solution and the status quo. A cost-benefit analysis can help evaluate and justify the feasibility and desirability of each alternative, and support the decision-making process. Calculating the total cost of ownership (TCO) is a step that involves estimating the direct and indirect costs associated with acquiring, operating, maintaining, and disposing of an asset or a system over its entire life cycle. A TCO calculation can help determine the long-term financial implications of investing in a new IDS solution, and compare it with other alternatives. Conducting a feasibility study is a step that involves assessing the technical, operational, legal, and economic aspects of implementing a project or an investment. A feasibility study can help identify and mitigate any potential issues or risks that may affect the success of the project or investment, and provide recommendations for improvement

Identifying critical functions is the foundation for understanding which processes must be prioritized for protection and recovery.
"Identifying and prioritizing critical business functions is essential to the success of the BIA."
- CISM Review Manual 15th Edition, Chapter 3: Information Security Program Development and Management, Section: Business Impact Analysis*