This control is best implemented through system configuration because it can be enforced automatically by setting a parameter in the network operating system or directory service. This ensures that temporary workers do not have access to the network beyond their authorized period, and reduces the risk of unauthorized or malicious use of their accounts12.
References1: Configuration and Change Management - CISA2: What is IT Governance? - Definition from Techopedia

The most important thing for an IS auditor to validate when auditing network device management is that all devices have current security patches assessed. This is because security patches are essential for fixing known vulnerabilities and preventing unauthorized access, data breaches, or denial-of-service attacks on the network devices. If the network devices are not patched regularly, they may expose the network to various cyber threats and compromise the confidentiality, integrity, and availability of the network services and data12.
Devices cannot be accessed through service accounts is not the most important thing to validate because service accounts are typically used for automated tasks or processes that require privileged access to network devices. Service accounts can be secured by using strong passwords, limiting their permissions, and monitoring their activities. However, service accounts alone do not protect the network devices from external or internal attacks that exploit unpatched vulnerabilities3.
Backup policies include device configuration files is not the most important thing to validate because backup policies are mainly used for restoring the network devices in case of failure, disaster, or corruption. Backup policies can help with recovering the network functionality and data, but they do not prevent the network devices from being compromised or attacked in the first place. Backup policies should be complemented by security policies that ensure the network devices are patched and protected4.
All devices are located within a protected network segment is not the most important thing to validate because network segmentation is a technique that divides the network into smaller subnets or zones based on different criteria, such as function, security level, or access control. Network segmentation can help isolate and contain the impact of a potential attack on a network device, but it does not prevent the attack from happening.
Network segmentation should be combined with security patching and other security measures to ensure the network devices are secure.

Integration testing is the best way to ensure that an application is performing according to its specifications, because it tests the interaction and compatibility of different modules or components of the application. Unit testing, pilot testing and system testing are also important, but they do not cover the whole functionality and integration of the application as well as integration testing does. References: CISA Review Manual (Digital Version)1, Chapter 4, Section 4.2.3

A transaction processing system (TPS) is a system that captures, processes, and stores data related to business transactions1. A general ledger is a system that records the financial transactions of an organization in different accounts2. An interface is a connection point between two systems that allows data exchange3. A system fix is a change or update to a system that resolves a problem or improves its functionality4.
The IS auditor should recommend to perform periodic reconciliations to validate the interface between the TPS and the general ledger is working in the future. A reconciliation is a process of comparing and verifying the data in two systems to ensure accuracy and consistency1. By performing periodic reconciliations, the IS auditor can detect and correct any errors or discrepancies in the data, such as duplicate transactions, missing transactions, or incorrect amounts. This way, the IS auditor can ensure the reliability and integrity of the financial data in both systems.
The other options are not as effective as periodic reconciliations to validate the interface. System owner sign- off for the system fix is a form of approval that indicates the system owner agrees with the change and its expected outcome4. However, this does not guarantee that the system fix will work as intended or prevent future errors. Conducting functional testing is a process of verifying that the system performs its intended functions correctly and meets its requirements4. However, this is usually done before or after the system fix is implemented, not on an ongoing basis. Improving user acceptance testing (UAT) is a process of evaluating whether the system meets the needs and expectations of the end users4. However, this is also done before or after the system fix is implemented, not on an ongoing basis. Therefore, option A is the correct answer.
References:
Transaction Interface: Organization, Process, and System
Validation of Interfaces - Ensuring Data Integrity and Quality across Systems Oracle Payments Implementation Guide Receiving Transactions Inserted Into Interface Table as BATCH And PENDING Are Not Processed By Receiving Transaction Processor What Is a Transaction Processing System (TPS)? (Plus Types)