When using @WebMvcTest, the security configurations from the main application apply by default. You should be aware that the security settings of the application being tested will be active, and you may need to adjust your tests accordingly. Options 1, 3, and 4 do not accurately describe how security configurations work in @WebMvcTest.

To enable method-level security in Spring Security, you need to use the @EnableGlobalMethodSecurity annotation on a configuration class. It's used to activate annotations like @Secured, @PreAuthorize, etc., for method-level security.

In Spring Boot, to define a global default exception handler for unhandled exceptions, you use the @ControllerAdvice annotation. This annotation allows you to define a class that can be applied globally to handle exceptions across all controllers. It provides a way to centralize exception handling in your application.

You can change the default port number of the embedded web server in a Spring Boot application by modifying the server.port property in the application.properties file. This property allows you to specify the desired port number. Spring Boot automatically configures the embedded web server (e.g., Tomcat) based on this property, making it a straightforward way to control the server's port.

To address the requirement of providing custom error responses from individual controllers while maintaining the functionality of the global exception handler in a complex Spring Boot application, you can implement individual @ExceptionHandler methods in each controller. This approach allows for controller-specific exception handling while still benefiting from the global exception handler for unhandled cases. The other options may not meet the requirement effectively.

Reactive Streams in Spring Boot offer an "asynchronous approach" to handle the flow of data between the producer and consumer. Reactive Streams are designed for asynchronous, non-blocking processing of data and provide a standardized way to deal with data streams, making it easier to handle data in a reactive and efficient manner.

Garbage Collection optimization is significant in Spring Boot because it can reduce memory usage. Inefficient garbage collection can lead to increased memory consumption, longer pauses, and application slowdowns. By optimizing garbage collection settings and strategies, you can reduce memory overhead, minimize pause times, and improve overall application performance. Garbage Collection does not directly impact database performance or network communication speed in Spring Boot applications.

In Spring Boot, the module that enables the development of reactive applications is spring-webflux. This module provides a foundation for building reactive, non-blocking applications. It includes support for creating reactive RESTful services and interacting with reactive data sources. Reactive programming is particularly useful for handling high concurrency and low latency scenarios.

To integrate a custom authentication provider in Spring Security, you should add a custom AuthenticationProvider bean to the Spring application context and configure it in the security configuration. This allows Spring Security to use your custom logic for authentication. Options 1 and 4 are not correct; they do not represent the standard way of integrating custom authentication providers. Option 2 is also incorrect as authentication providers are typically configured programmatically, not in properties files.

Method-level security annotations in Spring Security are implemented by using @EnableMethodSecurity in a @Configuration class and applying @PreAuthorize and @PostAuthorize annotations on methods. Options 1 and 4 are incorrect, while Option 2 is partially correct but not the recommended approach.