A security policy refers to internal rules that govern how an organization protects critical system resources, such as industrial control systems (ICS). A security policy defines the objectives, scope, roles, responsibilities, and requirements for securing the ICS environment, as well as the procedures and guidelines for implementing, monitoring, and enforcing the security measures. A security policy also establishes the baseline for assessing and managing the security risks to the ICS, and for ensuring compliance with relevant standards, regulations, and best practices. A security policy is a key component of the ICS security program, and it should be documented, communicated, and reviewed regularly.

The other choices are not correct because:

A. Formal guidance. Formal guidance refers to external sources of information and recommendations that can help an organization improve its ICS security posture, such as standards, frameworks, guidelines, and best practices. Formal guidance is not an internal rule, but rather a reference that can be used to develop, implement, and evaluate the security policy and controls. For example, the ISA/IEC 62443 series of standards provide formal guidance on how to secure ICS from cyber threats1.

B. Legislation. Legislation refers to external laws and regulations that impose legal obligations and penalties on an organization for its ICS security performance, such as the NERC CIP standards for the electric sector2, or the EU NIS Directive for critical infrastructure operators3. Legislation is not an internal rule, but rather a compliance requirement that must be met by the organization. Legislation may also influence the security policy and controls, as the organization needs to align its security objectives and practices with the legal expectations and consequences.

D. Code of conduct. A code of conduct refers to a set of ethical principles and values that guide the behavior and decision-making of an organization and its employees, such as honesty, integrity, respect, and accountability. A code of conduct is not an internal rule for protecting critical system resources, but rather a general norm for conducting business and maintaining a positive reputation. A code of conduct may also support the security policy and culture, as it can foster a sense of responsibility and trust among the ICS stakeholders.