The information security manager's primary focus during the development of a critical system storing highly confidential data should be ensuring the amount of residual risk is acceptable. Residual risk is the level of cyber risk remaining after all the security controls are accounted for, any threats have been addressed and the organization is meeting security standards. It's the risk that slips through the cracks of the system. For a critical system storing highly confidential data, the residual risk should be as low as possible, and within the organization's risk appetite and tolerance. The information security manager should monitor and review the residual risk throughout the system development life cycle, and ensure that it is communicated and approved by the appropriate stakeholders. The other options are not the primary focus, although they may be part of the security objectives and activities. Reducing the number of vulnerabilities detected is a desirable outcome, but it does not necessarily mean that the residual risk is acceptable, as some vulnerabilities may have a higher impact or likelihood than others. Avoiding identified system threats is a preventive measure, but it does not account for unknown or emerging threats that may pose a residual risk to the system. Complying with regulatory requirements is a mandatory obligation, but it does not guarantee that the residual risk is acceptable, as regulations may not cover all aspects of security or reflect the specific context and needs of the organization.

The BEST indication that an organization has a mature information security culture is when its staff consistently consider risk in making decisions. When an organization's staff understands the risks associated with their actions and are empowered to make risk-informed decisions, it indicates that the organization has a mature information security culture.
According to the Certified Information Security Manager (CISM) Study Manual, "A mature information security culture exists when the people within the organization understand and appreciate the risks associated with information and technology and when they take steps to manage those risks on a daily basis." While information security training, documented information security policies, and regular interaction between the chief information security officer (CISO) and the board are all important components of a mature information security culture, they are not sufficient on their own. It is only when staff consistently consider risk in making decisions that an organization's information security culture can be considered mature.
Reference:
Certified Information Security Manager (CISM) Study Manual, 15th Edition, Pages 151-152.