Explanation
The OAuth protocol is a standard way to authorize access to web resources. By configuring a connected app with an authorization endpoint of the API gateway, Salesforce can obtain an access token from the API gateway and use it to invoke the external API securely. This avoids the need to manage certificates or user credentials for authentication2 References: 1: Data Virtualization Pattern 2: OAuth 2.0 Web Server Authentication Flow

Explanation
Option B is a correct finding, because it means that the company can use their cloud-based ETL tools to extract data from the legacy systems via the APIs exposed by the on-premise middleware, transform the data as needed, and load the data into Salesforce using queues. Queues are a way of managing asynchronous operations, such as bulk data loading, by placing them in a queue and executing them when resources are available1.
Option C is also a correct finding, because it means that the company can expose their legacy data to the cloud via their on-premise middleware APIs. APIs are a way of enabling communication and data exchange between different systems using standard protocols and formats2. By providing APIs to access their legacy data, the company can use any cloud-based tool or application that can consume those APIs to retrieve and manipulate the data.
Option A is not a correct finding, because it means that the company cannot access their legacy data from the cloud, which prevents them from using their cloud-based ETL tools or any other cloud-based integration solution. If only on-premise systems are allowed to access the legacy systems, then the company would need to use an on-premise integration tool or application to copy the data to Salesforce.
Option D is not a correct finding, because it does not affect the ability to copy legacy data in Salesforce.
Queues are useful for on-premise integration, but they are not necessary for copying data to Salesforce. The company can use other methods, such as direct API calls or batch processes, to load data into Salesforce without using queues.
References: 1: Queueable Apex | Apex Developer Guide | Salesforce Developers 2: API Basics | SOAP API Developer Guide | Salesforce Developers

Explanation
Middleware should coordinate request delivery and payment processing, and Payment System should process a payment request only once. This solution ensures that each payment request is delivered to the Payment System in a reliable and consistent manner, and avoids duplicate or conflicting updates to the same Payment Request record. Middleware can act as a mediator between the Data Entry Point and the Payment System, and implement logic to handle errors, retries, and acknowledgments. Payment System can use the globally unique identifier to check if a payment request has already been processed, and avoid processing it again.
References: Certification - Integration Architect - Trailhead, [Integration Patterns and Practices]

Explanation
Streaming API is the best option for sending real-time notifications of changes to Salesforce data. Streaming API uses a publish-subscribe model to push relevant data to subscribers without polling. Streaming API supports PushTopic events, which are based on SOQL queries that define the data changes to listen for. The affiliate company can subscribe to a PushTopic event on the NTO Salesforce org and receive notifications whenever the data that matches the query changes. This way, the affiliate company can be informed of any updates to the opportunities in the NTO Salesforce instance.

Explanation
Push Topic Events can define a custom payload, which is one of the factors that should be considered when choosing a Streaming API implementation. Push Topic Events are based on SOQL queries that define the data changes to be notified of. You can specify the fields to be returned in the event message by using the SELECT clause in the query. This allows you to customize the payload according to your needs2 References: Create a PushTopic