a. The servlet container (i.e. web server) calls the service() method to handle requests coming from the client.
b. Each time the server receives a request for a servlet, the server spawns a new thread and calls service.
c. The service() method checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet, doPost, doPut, doDelete, etc. methods as appropriate.
d. All of the above.