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.