Explanation
The first step to gain approval for outsourcing to address a security gap is to perform a cost-benefit analysis, because it helps to evaluate the feasibility and viability of the outsourcing option and compare it with other alternatives. A cost-benefit analysis is a method of estimating and comparing the costs and benefits of a project or a decision, in terms of financial, operational, and strategic aspects. A cost-benefit analysis can help to:
Identify and quantify the expected costs and benefits of outsourcing, such as the initial and ongoing expenses, the potential savings and revenues, the quality and efficiency of the service, the risks and opportunities, and the alignment with the business objectives and requirements Assess and prioritize the criticality and urgency of the security gap, and the impact and likelihood of the related threats and vulnerabilities Determine the optimal level and scope of outsourcing, such as the type, duration, and frequency of the service, the roles and responsibilities of the parties involved, and the performance and security standards and metrics Justify and communicate the rationale and value proposition of outsourcing, and provide evidence and support for the decision making process Establish and document the criteria and process for selecting and evaluating the outsourcing provider, and the contractual and legal terms and conditions A cost-benefit analysis should be performed before submitting a funding request to senior management, because it can help to demonstrate the need and the return on investment of the outsourcing project, and to secure the budget and the resources. A cost-benefit analysis should also be performed before beginning due diligence on the outsourcing company, because it can help to narrow down the list of potential candidates and to focus on the most relevant and suitable ones. Collecting additional metrics may be a part of the cost-benefit analysis, but it is not the first step, because it requires a clear definition and understanding of the objectives and scope of the outsourcing project.
References = CISM Review Manual, 16th Edition, ISACA, 2021, pages 173-174, 177-178.

The explanation given in the manual is:
The information security manager's knowledge of the business is the most critical factor for information security program success because it enables him or her to align security objectives with business goals and communicate effectively with senior management and other stakeholders. The other choices are important elements of an information security program but not as critical as the information security manager's knowledge of the business.
An information security program is a set of policies, procedures, standards, guidelines, and tools that aim to protect an organization's information assets from threats and ensure compliance with laws and regulations.
An information security manager is a professional who oversees and coordinates the implementation and maintenance of an information security program. An information security manager should have a good understanding of the business environment, culture, strategy, processes, and needs of an organization to ensure that security supports its objectives.

Explanation
The criticality to business should always drive the decision. Regulatory requirements could be more flexible than business needs. The financial value of an asset could not correspond to its business value. While a consideration, IT resource availability is not a primary factor.