Understanding HTTP status codes is vital when working with Web APIs because they provide valuable information about the success or failure of API requests. These codes convey details such as whether a request was successful, encountered an error, or requires further action. This knowledge helps developers diagnose and handle issues, ensuring a more reliable and effective interaction with Web APIs.

An API Gateway plays a critical role in API security. Option A is correct because it acts as a central entry point for APIs, enabling authentication, authorization, logging, and more. It does not primarily handle encryption of API responses, expose all APIs to the public, or manage database access control.

To validate an API's ability to handle 10,000 requests per minute, it's essential to simulate real-world conditions. Option 3 suggests simulating 10,000 concurrent users with gradual ramp-up, which is an appropriate approach for load testing. This approach helps identify how the API performs under the specified load, ensuring it meets the requirement. Option 1, with 100,000 virtual users, may not represent the real use case, and option 2 is inadequate for this requirement. Option 4, manually increasing load, lacks automation and may not provide accurate results.

When setting up monitoring for an API, it's important to consider factors such as monitoring response times and error rates. This enables early detection of issues, performance analysis, and ensuring the API's reliability and availability.

If an API key is accidentally leaked, the best practice is to immediately generate a new API key and update it in the application and relevant services. This prevents unauthorized access and potential security breaches. Continuing to use the same key without taking action could lead to security risks. Contacting the API provider may also be necessary, but the primary step is to update the key on your side.

The "access token" in OAuth 2.0 is a short-lived token used for user authentication. It provides the client (application) with limited access to a protected resource on behalf of the resource owner (user) after the user has granted permission. This token is crucial for securing APIs and ensuring authorized access to resources.

When using JWTs for authorization, potential security concerns include vulnerability to replay attacks. Since JWTs are self-contained, there's a risk of an attacker reusing a valid token to impersonate a user or gain unauthorized access. Implementing proper measures to prevent replay attacks is crucial when using JWTs in an authentication and authorization system.

Data encryption in APIs is crucial for privacy and compliance. It helps secure sensitive data from unauthorized access by encrypting it, making it unreadable without the proper decryption key. This ensures that sensitive information remains private and compliant with data protection regulations, enhancing the trust of API users.

When integrating with legacy systems, factors to consider for the API architectural style include compatibility with existing systems. SOAP APIs are often suitable for legacy systems due to their strict contract-based approach. RESTful APIs can be lightweight and suitable for some legacy systems. gRPC is a modern, high-performance option but may not be ideal for legacy integration. GraphQL APIs offer flexibility but may not align with legacy systems with fixed data structures.

For a highly dynamic application with frequently changing data requirements, a GraphQL API would be most suitable. GraphQL allows clients to request precisely the data they need, reducing over-fetching or under-fetching. It adapts well to changing requirements as clients can modify their queries without the need for server-side changes. RESTful APIs are more rigid in terms of data structure, and SOAP APIs are more suited for static data models. WebSocket APIs offer real-time communication but may not be ideal for highly dynamic data.