The MOST important reason to monitor implemented controls is to ensure the controls:

 Importance of Monitoring Controls:

Monitoring implemented controls is a critical aspect of risk management and audit practices. The primary goal is to ensure that the controls are functioning as intended and effectively mitigating identified risks.

 Effectiveness and Risk Management:

Controls are put in place to manage risks to acceptable levels, as determined by the organization's risk appetite and risk management framework. Regular monitoring helps in verifying the effectiveness of these controls and whether they continue to manage risks appropriately.

References from the ISA 315 standard emphasize the importance of evaluating and monitoring controls to ensure they address the risks they were designed to mitigate​​​​.

 Other Considerations:

While enabling IT operations to meet agreed service levels (B) and mitigating regulatory compliance risks (C) are important, they are secondary to the primary purpose of ensuring controls are effective in managing risk.

Effective risk management encompasses meeting service levels and compliance, but these are outcomes of having robust, effective controls.

 Conclusion:

Therefore, the most important reason to monitor implemented controls is to ensure they are effective and manage risk to the desired level.