Periodically reviewing log files is the most effective way to detect intrusion attempts from outside the organization, as they can provide evidence of unauthorized access attempts, source IP addresses, timestamps and other relevant information. Using smart cards with one-time passwords or installing biometrics-based authentication can prevent unauthorized access, but not detect it. Configuring the router as a firewall can block unwanted traffic, but not log it. References: ISACA, CISA Review Manual, 27th Edition, 2018, page
361

The auditor's best recommendation prior to go-live is to conduct a mock conversion test. This is because a mock conversion test can help to verify the accuracy, completeness, and validity of the data conversion process. A mock conversion test can also help to identify and resolve any issues or errors before the actual conversion takes place. A mock conversion test can also provide assurance that the converted data meets the business requirements and expectations. References:
* CISA Review Manual (Digital Version), Chapter 3, Section 3.3.21
* CISA Online Review Course, Domain 2, Module 2, Lesson 22

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