The lack of system documentation should be of most concern to an IS auditor reviewing the information systems acquisition, development, and implementation process. This is because system documentation is a vital source of information that describes the system's purpose, functionality, design, architecture, testing, deployment, operation, and maintenance. System documentation helps the IS auditor to understand and evaluate the system's quality, performance, security, compliance, and alignment with the business requirements and objectives. Without system documentation, the IS auditor may not be able to perform a thorough and effective audit of the system, as well as identify any issues or risks that may affect the system's reliability or integrity12.
Data owners are not trained on the use of data conversion tools is not the most concerning issue, although it may indicate a lack of user readiness or competence for the system implementation. Data conversion tools are software applications that help users to transform data from one format or structure to another, such as from legacy systems to new systems. Data owners are users who have the responsibility and authority to manage and control the data within their domain. Data owners should be trained on how to use data conversion tools to ensure that the data is accurately and securely transferred to the new system, as well as to avoid any data loss, corruption, or inconsistency. However, data owners are not the only users who need training for the system implementation, and data conversion tools are not the only tools that need training34.
A post-implementation lessons-learned exercise was not conducted is not the most concerning issue, although it may indicate a lack of continuous improvement or learning culture for the system development and implementation process. A post-implementation lessons-learned exercise is a meeting or a session that takes place after the completion of a system implementation project, where the project team and stakeholders discuss and document the successes and failures of the project, as well as identify any best practices or areas for improvement for future projects. A post-implementation lessons-learned exercise can help to enhance the project management skills, knowledge, and performance of the project team and stakeholders, as well as to avoid repeating the same mistakes or problems in future projects56.
System deployment is routinely performed by contractors is not the most concerning issue, although it may pose some challenges or risks for the system implementation process. System deployment is the final stage of the system development life cycle (SDLC), where the system is installed and configured on the target environment and made available for use by end-users. System deployment can be performed by internal staff or external contractors, depending on the availability, expertise, and cost of resources. System deployment by contractors may offer some benefits such as faster delivery, lower cost, or higher quality than internal staff. However, system deployment by contractors may also introduce some risks such as loss of control, dependency, or security breaches over the system implementation process

The organization is primarily responsible for the security configurations of the deployed application's operating system when migrating its HR application to an Infrastructure as a Service (IaaS) model in a private cloud. This is because in an IaaS model, the cloud provider is responsible for the security of the underlying infrastructure that they lease to their customers, such as servers, storage, and networks, while the customer is responsible for the security of the areas of the cloud infrastructure over which they have control, such as operating systems, middleware, and applications. Therefore, the organization needs to ensure that the operating system is properly configured, patched, hardened, and monitored to protect the HR application from unauthorized access or malicious attacks.
The other options are not primarily responsible for the security configurations of the deployed application's operating system. The cloud provider's external auditor is not responsible for any security configurations, but rather for verifying and reporting on the cloud provider's compliance with relevant standards and regulations.
The cloud provider is responsible for the security of the underlying infrastructure, but not for the operating system or any software installed on it by the customer. The operating system vendor is responsible for providing updates and patches for the operating system, but not for configuring or securing it according to the customer's needs.
References:
* 11: What Is IaaS (Infrastructure As A Service)? - Forbes
* 12: What is Shared Responsibility Model? - Check Point Software
* 13: Who Is Responsible for Cloud Security? - Security Intelligence

The greatest concern with the IT department and internal IS audit function both reporting to the CIO is the potential compromise of IS audit independence. Auditor independence refers to the impartiality and objectivity of an auditor in conducting an audit, free from conflicts of interest and bias1. It is crucial for ensuring the quality and reliability of financial reporting1. If the IS audit function reports to the CIO, who also oversees the IT department, it could create a conflict of interest that might compromise the impartiality and objectivity of the IS audit function.
References:
Auditor Independence - What is it, Rules, Importance, Examples

The best source of information to determine the required level of data protection on a file server is the data classification policy and procedures, which define the criteria and methods for classifying data according to its sensitivity, value, and criticality, and specify the appropriate security measures and controls for each data category. Data classification policy and procedures help to ensure that data is protected in proportion to its importance and risk exposure. Access rights of similar file servers, previous data breach incident reports, and acceptable use policy and privacy statements are not sufficient or reliable sources of information to determine the required level of data protection on a file server, as they do not provide clear and consistent guidance on how to classify and protect data. References: CISA Review Manual (Digital Version), Chapter 5: Protection of Information Assets, Section 5.1: Information Asset Security Framework