An advanced persistent threat (APT) is a stealthy and sophisticated attack that aims to compromise and maintain access to a target network or system over a long period of time, often for espionage or sabotage purposes. APTs are difficult to detect by conventional security tools, such as antivirus or firewalls, that rely on signatures or rules to identify threats. Therefore, the best way to monitor for APTs is to search for anomalies in the environment, such as unusual network traffic, user behavior, file activity, or system configuration changes, that may indicate a compromise or an ongoing attack. References: https://www.isaca.
org/credentialing/cism https://www.nist.gov/publications/information-security-handbook-guide-managers

A monitoring system for a critical application can help detect and prevent incidents that could affect the availability, integrity, and confidentiality of the application and its data. The impact of downtime could include loss of revenue, reputation, customer satisfaction, and regulatory compliance. Therefore, the cost of implementing the system should be justified by the potential savings from avoiding or minimizing these impacts.
References = CISM Review Manual, 15th Edition, page 173; An Introduction to Metrics, Monitoring, and Alerting; Business-critical applications: What are they and how do you protect them from cyberattack?

An information security strategy is the most important element to have in place as a basis for developing an effective information security program that supports the organization's business goals. An information security strategy is a high-level plan that defines the vision, mission, objectives, scope, and principles of information security for the organization1. It also aligns the information security program with the organization's strategy, culture, risk appetite, and governance framework2. An information security strategy provides the direction, guidance, and justification for the information security program, and ensures that the program is consistent, coherent, and comprehensive3. An information security strategy also helps to prioritize the information security initiatives, allocate the resources, and measure the performance and value of the information security program4.
The other options are not as important as an information security strategy, because they are either derived from or dependent on the strategy. Metrics are used to drive the information security program, but they need to be based on the strategy and aligned with the goals and objectives of the program. Information security policies are the rules and standards that implement the information security strategy and define the expected behavior and responsibilities of the stakeholders. A defined security organizational structure is the way the information security roles and functions are organized and coordinated within the organization, and it should reflect the strategy and the governance model. References = 1: CISM Review Manual 15th Edition, Chapter 1, Section 1.1 2: CISM Review Manual 15th Edition, Chapter 1, Section 1.2 3: CISM Review Manual 15th Edition, Chapter 1, Section 1.3 4: CISM Review Manual 15th Edition, Chapter 1, Section 1.4 : CISM Review Manual 15th Edition, Chapter 1, Section 1.5 : CISM Review Manual 15th Edition, Chapter 1, Section 1.6 :
CISM Review Manual 15th Edition, Chapter 1, Section 1.7

The best first step is to assess the control state to determine why it is ineffective and whether adjustments, replacements, or compensating controls are needed.
"Regularly assess the effectiveness of security controls to ensure they are providing the intended level of protection."
- CISM Review Manual 15th Edition, Chapter 3: Information Security Program Development and Management, Section: Control Monitoring and Assessment* ISACA practice questions stress that assessing the control state comes before taking further action.