The first step in auditing a data communication system is to determine the business use and types of messages to be transmitted. This is because the auditor needs to understand the purpose, scope, and objectives of the data communication system, as well as the nature, volume, and sensitivity of the data being transmitted. This will help the auditor to identify the risks, controls, and audit criteria for the data communication system.
Traffic volumes and response-time criteria, physical security for network equipment, and the level of redundancy in the various communication paths are important aspects of a data communication system, but they are not the first step in auditing it. They depend on the business use and types of messages to be transmitted, and they may vary according to different scenarios and requirements. References: CISA Review Manual (Digital Version), [ISACA Auditing Standards]

A three-way match is a process of verifying that a purchase order, a goods receipt and an invoice are consistent before making a payment1. A three-way match ensures that the organization only pays for the goods or services that it ordered and received, and that the prices and quantities are accurate. A three-way match can prevent errors, fraud and overpayments in the accounts payable process.
An IS auditor should use a purchase order when verifying a three-way match has occurred in an enterprise resource planning (ERP) system. A purchase order is a document that authorizes a purchase transaction and specifies the items, quantities, prices and terms of the order2. A purchase order is the first document in the three-way match process, and it serves as the basis for comparing the goods receipt and the invoice. An IS auditor can use a purchase order to check if the ERP system has correctly recorded, matched and approved the three documents before making a payment.
The other options are not as useful for verifying a three-way match. A bank confirmation is a document that verifies the balance and activity of a bank account3. A bank confirmation can be used to confirm that a payment has been made or received, but it does not provide information about the details of the purchase transaction or the three-way match process. A goods delivery notification is a document that informs the buyer that the goods have been shipped or delivered by the seller4. A goods delivery notification can be used to track the status of the delivery, but it does not provide information about the quantity or quality of the goods or the invoice amount. A purchase requisition is a document that requests authorization to purchase goods or services from a specific supplier2. A purchase requisition can be used to initiate the purchasing process, but it does not provide information about the actual purchase order, goods receipt or invoice.
References:
Bank Confirmation - Overview, How It Works, Importance3
What is Goods Delivery Note? | Definition & Example4
What Is Three-Way Matching & Why Is It Important? | NetSuite1
Enterprise Resource Planning (ERP) - Definition, Types, Uses2

The most reliable way for an IS auditor to evaluate the operational effectiveness of an organization's data loss prevention (DLP) controls is to verify that confidential files cannot be transmitted to a personal USB device. This is because DLP controls are designed to prevent the loss, leakage or misuse of sensitive data through breaches, ex-filtration transmissions and unauthorized use1. A personal USB device is a common way for data to be stolen or compromised, as it can bypass network security measures and allow unauthorized access to confidential files. Therefore, testing the DLP controls by attempting to copy or transfer confidential files to a personal USB device can provide a direct and objective evidence of whether the DLP controls are working as intended or not.
The other options are less reliable ways for an IS auditor to evaluate the operational effectiveness of an organization's DLP controls. Reviewing data classification levels based on industry best practice is a way to assess the adequacy of the organization's data protection policies, but it does not measure how well the DLP controls are implemented or enforced in practice. Verifying that current DLP software is installed on all computer systems is a way to check the technical configuration of the DLP solution, but it does not test how well the DLP software detects and prevents data loss incidents in real scenarios. Conducting interviews to identify possible data protection vulnerabilities is a way to gather qualitative information from stakeholders, but it does not provide quantitative or empirical data on the actual performance of the DLP controls.
References:
What is Data Loss Prevention (DLP)? [Guide] - CrowdStrike