To understand which component executes EJB components, we need to have knowledge about the architecture of Enterprise JavaBeans (EJB) and the roles of different servers in the execution process.

EJB components are server-side components that are used to implement business logic in Java EE applications. They are executed by an EJB container, which is responsible for managing the lifecycle, deployment, and execution of EJB components.

Now let's go through each option and explain why it is right or wrong:

A. a web server: This option is incorrect. A web server is responsible for handling HTTP requests and serving web pages. It does not execute EJB components.

B. an EJB container: This option is correct. An EJB container is specifically designed to execute EJB components. It provides services such as object pooling, concurrency management, security, and transaction management. The EJB container ensures that EJB components are executed in a managed environment.

C. an application server: This option is also correct. An application server is a runtime environment that provides various services to execute enterprise applications. It includes an EJB container as one of its components. The application server manages the execution of EJB components along with other components of the application.

D. a database server: This option is incorrect. A database server is responsible for storing and retrieving data. It does not execute EJB components.

Therefore, the correct answer is:

The Answer is: B. an EJB container