The @SpringBootApplication annotation in Spring Boot is a convenience annotation that adds all of the following: @Configuration, @EnableAutoConfiguration, and @ComponentScan. Thus, it enables component scanning, allowing Spring to automatically discover and register beans. This is crucial for allowing the Spring context to be aware of all the components, services, repositories, etc. available in the project.

To implement the client credentials grant in a Spring Boot application, the client must send a request to the token endpoint with the client_credentials grant type. This grant type is used when the client, typically a service or application, needs to authenticate itself directly with the authorization server to obtain an access token. The other options are different OAuth2 grant types used for various scenarios.

To ensure fault tolerance and resilience in Spring Cloud microservices, you should implement Circuit Breaker patterns using tools like Hystrix. This helps prevent cascading failures and allows graceful degradation when a service is experiencing issues.

The primary role of a Resource Server in OAuth2 is to protect and serve protected resources. It validates access tokens presented by clients and enforces access control to ensure that only authorized clients can access protected resources. It does not generate access tokens (which is the responsibility of the Authorization Server), authenticate users, or store user credentials or data.

In OAuth2, the "Resource server" is responsible for serving protected resources to the client after successful authentication and authorization. It validates access tokens and ensures that the client has the necessary permissions to access the requested resources.

In a Spring Boot e-commerce application, the best approach to handle exceptions during inventory updates (Option 4) is to implement a centralized exception handling strategy with custom exception classes. This approach ensures proper rollback mechanisms and data integrity. Using try-catch blocks (Option 1) for local handling is not recommended for such critical operations. Logging failures and returning generic messages (Option 2) is insufficient for maintaining data integrity. Propagating exceptions (Option 3) without proper handling is also not ideal.

You can handle exceptions at the @RestController level by using @ExceptionHandler methods within the controller itself. This approach is different from using @ControllerAdvice, which is used for global exception handling across the application. @ControllerAdvice allows you to define exception handling methods that can be reused across multiple controllers, while @ExceptionHandler within the controller is specific to that controller.

To enable a service to register itself with Eureka Server in Spring Cloud, you can annotate the service class with @EnableDiscoveryClient. This annotation allows the service to participate in service discovery.

To resolve ambiguity in autowiring of beans, you can use the @Qualifier annotation to specify the bean name or qualifier explicitly. This approach ensures that the correct bean is injected. The @Primary annotation designates a primary bean, which can also help resolve ambiguity. The other options don't directly address ambiguity resolution.

Option 3 is the recommended approach. Tools like WireMock and MockServer allow you to create mock endpoints for external APIs, ensuring that actual API calls are not made during testing.