A post-implementation review (PIR) of a newly modified IT application focuses on ensuring that the system meets business and security requirements effectively. The sufficiency of implemented controls (A) is the most critical aspect because it ensures that security, operational, and compliance controls are functioning correctly.
These controls include access controls, data integrity checks, and audit logs to prevent unauthorized access, data corruption, or security breaches.
Other options:
Resource management plan (B) is important for project management but is not the primary concern for an IS auditor in a post-implementation review.
Updates required for end-user manuals (C) are necessary for usability but do not impact the security or operational integrity of the system.
Rollback plans for changes (D) are important for change management but are typically assessed before deployment, not in a PIR.
Reference: ISACA CISA Review Manual, IT Governance and Management of IT

The risk assessment process involves identifying the information assets that are at risk, analyzing the threats and vulnerabilities that could affect them, evaluating the impact and likelihood of a risk event, and determining the appropriate controls to mitigate the risk. The first step is to identify the information assets, as they are the objects of protection and the basis for the rest of the process. Without knowing what assets are at risk, it is not possible to assess their value, exposure, or protection level. References: ISACA Frameworks:
Blueprints for Success

During an audit of a reciprocal disaster recovery agreement between two companies, the IS auditor would be most concerned with the allocation of resources during an emergency. A reciprocal disaster recovery agreement is an arrangement by which one organization agrees to use another's resources in the event of a business continuity event or incident. The IS auditor would need to ensure that both parties have clearly defined their roles and responsibilities, their resource requirements, their priority levels, their communication channels, and their escalation procedures in case of a disaster. The IS auditor would also need to verify that both parties have tested their agreement and have updated it regularly to reflect any changes in their business environments. The frequency of system testing is not as critical as the allocation of resources during an emergency, because system testing can be performed periodically or on demand, while resource allocation is a dynamic and complex process that requires careful planning and coordination. The differences in IS policies and procedures are not as critical as the allocation of resources during an emergency, because both parties can agree on common standards and protocols for their disaster recovery operations, or they can adapt their policies and procedures to suit each other's needs. The maintenance of hardware and software compatibility is not as critical as the allocation of resources during an emergency, because both parties can use compatible or interoperable systems, or they can use virtualization or cloud computing technologies to overcome any compatibility issues. References: ISACA CISA Review Manual 27th Edition, page 281