The finding that should be of greatest concern to an IS auditor reviewing data conversion and migration during the implementation of a new application system is that unauthorized data modifications occurred during conversion. Data conversion and migration is a process that involves transferring data from one system to another, ensuring its accuracy, completeness, integrity, and usability. Unauthorized data modifications during conversion can result in data loss, corruption, inconsistency, or duplication, which can affect the functionality, performance, reliability, and security of the new system. Unauthorized data modifications can also have serious business implications, such as affecting decision making, reporting, compliance, customer service, and revenue. The IS auditor should verify that adequate controls are in place to prevent, detect, and correct unauthorized data modifications during conversion, such as access control, data validation, reconciliation, audit trail, and backup and recovery. The other findings (A, B and D) are less concerning, as they can be mitigated by documenting the change management process, restoring the backups of the old system and data from offline storage, or automating the data conversion process. References: CISA Review Manual (Digital Version), Chapter 3: Information Systems Acquisition, Development & Implementation, Section 3.4: System Implementation

The IS auditor's primary concern when auditing the proposed acquisition of new computer hardware is that a clear business case has been established. A business case is a document that justifies the need, feasibility, and benefits of a proposed project or investment. A clear business case can help to ensure that the acquisition of new computer hardware is aligned with the organization's goals, objectives, and requirements, and that it provides value for money and return on investment. The other options are not as important as establishing a clear business case, as they do not address the rationale or justification for acquiring new computer hardware.
References: CISA Review Manual, 27th Edition, page 467

Explanation
A demilitarized zone (DMZ) is a network segment that is separated from the internal network and the external network, such as the internet, by firewalls or other security devices. A DMZ provides an extra layer of security for the organization's internal network by isolating the servers and services that need to be accessible to external users, such as a file server, from the rest of the network. A DMZ also prevents external users from accessing the internal network directly, as they have to go through two firewalls to reach it. Therefore, setting up a DMZ is an IS auditor's best recommendation to protect an organization from attacks when its file server needs to be accessible to external users12.
The other possible options are:
Enforce a secure tunnel connection: This means that the organization requires external users to establish a secure and encrypted connection, such as a virtual private network (VPN), to access its file server.
This can provide some level of security and privacy for the data transmission, but it does not protect the file server or the internal network from attacks if the connection is compromised or if the external users are malicious. Therefore, enforcing a secure tunnel connection is not an IS auditor's best recommendation to protect an organization from attacks when its file server needs to be accessible to external users3.
Enhance internal firewalls: This means that the organization improves the security and performance of its internal firewalls, which are devices that filter and control the network traffic between different segments of the network. This can provide some level of protection for the internal network from unauthorized or malicious access, but it does not protect the file server or the external network from attacks if the file server is exposed to the internet or if the external network is compromised. Therefore, enhancing internal firewalls is not an IS auditor's best recommendation to protect an organization from attacks when its file server needs to be accessible to external users4.
Implement a secure protocol: This means that the organization uses a secure and standardized protocol, such as Secure File Transfer Protocol (SFTP) or Secure Shell (SSH), to transfer files between its file server and external users. This can provide some level of security and integrity for the data transmission, but it does not protect the file server or the internal network from attacks if the protocol is exploited or if the external users are malicious. Therefore, implementing a secure protocol is not an IS auditor's best recommendation to protect an organization from attacks when its file server needs to be accessible to external users5. References: 1: What Is a DMZ Network and Why Would You Use It? | Fortinet 2:
Demilitarised zone (DMZ) | Cyber.gov.au 3: What Is VPN Tunneling? | Fortinet 4: Firewall - Wikipedia 5: Secure Shell - Wikipedia

The greatest concern when implementing firewalls on top of commercial operating systems is the potential presence of vulnerabilities that could undermine the security posture of the firewall platform itself. In most circumstances, when commercial firewalls are breached that breach is facilitated by vulnerabilities in the underlying operating system. Keeping all installation options available on the system further increases the risks of vulnerabilities and exploits. Using SSL for firewall administration (choice A) is important, because changes in user and supply chain partners' roles and profiles will be dynamic. Therefore, it is appropriate to maintain the firewall policies daily (choice B), and prudent to block all inbound traffic unless permitted (choice C).