The accountability for a business continuity program for business-critical systems is BEST assigned to the:

The accountability for a business continuity program for business-critical systems is best assigned to the CIO, because the CIO is responsible for the IT strategy, operations, and resources that support the business objectives and continuity. The other options are not as suitable as the CIO, because they do not have the same level of authority, expertise, or involvement in the IT function. The enterprise risk manager oversees the overall risk management process, but does not have direct control over the IT resources and activities. The CEO is ultimately accountable for the entire organization, but delegates the responsibility for IT to the CIO. The director of internal audit provides assurance and consulting services on the effectiveness of governance, risk management, and control processes, but does not have operational responsibility for IT or business continuity. References := Business Continuity Program Roles & Responsibilities, Who Should Manage the Business Continuity Program?