Explanation

In operational risk management governance, the board of directors has distinct responsibilities from senior management:

• Option A (Implementing operational risk management systems) - This is typically a responsibility of senior management, who execute the board's directives
• Option B (Develop governance structure) - While the board oversees governance, the actual development is usually delegated to senior management
• Option C (Assigning responsibilities to risk managers) - This is an operational function typically handled by senior management
• Option D (Periodically reviewing and approving the operational risk management framework) - This is a core responsibility of the board of directors

According to regulatory standards and best practices in operational risk management:

• The board of directors is responsible for oversight and approval of the risk management framework
• The board should periodically review and approve the operational risk management framework to ensure it remains effective and appropriate
• Senior management is responsible for implementation and day-to-day management of operational risk

Therefore, the board of directors should be responsible for periodically reviewing and approving the operational risk management framework, making Option D the correct answer.