The @RestController annotation is used in Spring Boot to define a RESTful web service endpoint. It indicates that the class is a controller responsible for handling HTTP requests and returning responses in a RESTful manner. This annotation is essential for building REST APIs in Spring Boot. The other options do not accurately describe the purpose of this annotation.

To effectively support both web applications and RESTful APIs in a Spring Boot project, it's best practice to create separate controllers and endpoints, using @Controller for web applications and @RestController for RESTful APIs. This approach ensures clear separation of concerns and allows for different response types and handling for each type of client.

The @Transactional annotation in Spring Data JPA is used to define transaction boundaries for the annotated method. It ensures that the method is executed within a transaction, and any changes made to the database are either committed or rolled back as a single unit of work. The other options do not accurately describe the purpose of @Transactional in Spring Data JPA.

To implement password rotation while maintaining security, you should follow best practices like using a strong hashing algorithm, enforcing a secure password policy, and implementing a scheduled task to rotate passwords. Storing expiration dates in plaintext or using weak hashing would compromise security.

If you want to specify that a configuration will be applied only if a specific class is present, you would use the @ConditionalOnClass annotation in Spring Boot. This annotation allows you to conditionally apply a configuration based on the presence of a specified class in the classpath. It helps in creating flexible and conditional configurations.

To optimize the performance of Spring Data JPA repositories when dealing with large datasets, it's crucial to write custom queries using the @Query annotation. This allows you to fine-tune the SQL queries and fetch only the necessary data, minimizing the overhead of retrieving large datasets. Other options may be relevant in different contexts, but they do not directly address the issue of optimizing performance with large datasets.

In Spring Boot, you can optimize configuration loading during testing by using the @Profile annotation in test classes. This allows you to specify which profiles should be active during testing, overriding the application's default profile. Option (1) is not the preferred way for testing. Option (4) is incorrect as it is used to set the default profile, not for testing purposes. Option (3) is incorrect; profiles can indeed be optimized for testing.

In Spring Boot, the @ConditionalOnBean annotation is used to create a condition based on the presence or absence of a specific bean in the application context. This allows you to configure certain components or behavior only if a particular bean is defined, making it a powerful tool for conditional configuration.

In Spring Data JPA, when the derived query mechanism is not sufficient, you can annotate a custom query method with the @Query annotation. This annotation allows you to define custom queries using JPQL or native SQL. There's no @CustomQuery annotation in Spring Data JPA, and the other options mentioned are not standard annotations for this purpose.

In OAuth2, the "Resource Owner Password Credentials" grant type is used by clients to exchange user credentials (username and password) directly for an access token. This grant type is typically used when the client and authorization server trust each other, and it's not suitable for public clients.