OAuth 2.0 is a widely adopted authentication strategy for securing APIs accessed by various clients. It provides security features such as token-based authentication, authorization, and is suitable for both web clients and services. JWT is a token format and not an authentication strategy on its own. Basic Authentication and API Keys have limitations in terms of security and scalability.

In Express.js, the all() method is used to handle all HTTP methods (GET, POST, PUT, DELETE, etc.) for a specific route. It is equivalent in functionality to the any() method. The use() and match() methods do not provide the same functionality.

The Event Loop in Node.js handles asynchronous tasks by adding them to a queue and executing them in a non-blocking manner. It ensures that asynchronous tasks are processed in the background without pausing the main thread, allowing Node.js to remain responsive. The other options do not accurately describe how asynchronous tasks are handled by the Event Loop.

To resolve conflicts between different versions of the same package required by different dependencies, you can manually specify the version of the package in your project's package.json file using the "dependencies" section. This allows you to enforce a specific version and avoid conflicts. Updating all dependencies may introduce compatibility issues and is not recommended. Switching to a different package manager or deleting the package are not typically the best solutions.

Integration testing focuses on the dependencies between different components of the application. It ensures that these components work together as expected. While interactions and behavior are important aspects, they are not the primary focus of integration testing. Functions are a narrower concept and do not encompass all aspects of integration.

To validate incoming request payloads in Express, it's recommended to use a library like express-validator. This library provides a convenient way to validate and sanitize user input, making it an essential tool for building secure and robust applications.

When determining the expiration time of a JWT (JSON Web Token), you need to balance security and usability. Setting it too short might lead to inconvenience, while setting it too long could be a security risk. It's important to find the right balance to protect the token's integrity. The other options don't provide a balanced approach.

The Bearer header is often used to pass the authentication token from the client to the server in HTTP requests when using JWTs for authentication. It is a common practice to include the JWT as a Bearer token in the Authorization header. The other options may not be standard headers for this purpose.

The primary difference is that OAuth 1.0 requires client registration, while OAuth 2.0 does not. OAuth 2.0 introduced a more streamlined and flexible approach to authorization. The other options describe differences but not the primary distinction.

In Express.js, the :id? in a route path like "/users/:id?" denotes that id is an optional parameter. This means that the id parameter may or may not be present in the URL, and the route will still match. The other options are not correct in this context (required would mean the parameter is mandatory, query is used for query parameters, and body is used for request bodies).