Admitting that entity and SLSBs are multithreaded components, while a bean instance is single threaded, I would go with a) and b). Am I right? However some containers like weblogic have special flags that could enable a SFSB to be multithreaded as well. Hence if this question comes from a bea exam, then one could consider c) as a possible answer. Confusing enough? Gosh I hate these questions :-)
I would think sesisonbean are single threaded and entity beans are multithreaded.
For SessionBeans, there will be several session bean instances in the pool and each will fullfill the request for each client business method call. After fullfilling the request of the client, the bean goes back to the pool. Since each bean is for each client, it is single threaded.
But for Entity bean, each bean can handle multiple request from the client, so it is multithreaded.