when not defining a commit option in jboss.xml I understand that commit option A is used as default.
when I manipulate an entity bean in a session bean(means a record in rdbms) does commit option A mean that only this instance of the session bean can manipulate the state of the entity bean an no one else at the same time?
No, that depends on the locking behavior of your database/sql combination. The commit options only tell entity beans how often to read their data from the db (+ some cache timing for B,C)