Final answer:
The common fixed role that exists in all databases is the public role. It is a default role included to provide base-level permissions deemed safe for all users. Other roles like db_owner, db_datareader, and db_ddladmin have specific permissions and exist in systems like SQL Server but are not automatically common to all databases.
Step-by-step explanation:
Common fixed roles are predefined in databases to simplify the management of user permissions. The public role is a special default role that exists in all databases. When a user is created, it's automatically given permissions associated with the public role. This role includes a base level of permissions that are considered safe for all users.
In SQL Server databases, there are several fixed database roles, but not all of them are common to all databases. The db_owner role, for example, can perform all configuration and maintenance activities on the database and can also drop the database. The db_datareader role can read all data from all user tables. db_ddladmin is another fixed role, but it is specifically associated with SQL Server and has rights to run Data Definition Language (DDL) commands.
The correct answer to this question is B. public since it is the only role that is a fixed, default role for all users in every database.