You will have to examine how the question is worded. By default 11g will create all three tablespaces. You can however create a database with only SYSTEM and SYSAUX. Technically speaking you can upgrade pre-10g database and not add SYSAUX, or remove the SYSAUX tablespace after the database is created and therefore have a database with only a SYSTEM tablespace, but you would be ill advised to do so and I really don't think they would look for that in an answer.
From the 11.2 Administration Guide:
The SYSAUX tablespace is always created at database creation.
From another section of the Administration Guide:
An auto-extending undo tablespace named UNDOTBS1 is automatically
created when you create the database with Database Configuration
Assistant (DBCA)...
If no undo tablespace is available, then the
instance starts without an undo tablespace, and undo data is written
to the SYSTEM tablespace. You should avoid running in this mode.
From the 10.2 Concepts Guide:
...the SYSAUX tablespace is always created during database creation or
database upgrade...
During normal database operation, the Oracle database server does not
allow the SYSAUX tablespace to be dropped or renamed. Transportable
tablespaces for SYSAUX is not supported.
Note: If the SYSAUX tablespace is unavailable, such as due to a media
failure, then some database features might fail.
From the 10.1 SQL Reference:
You cannot drop the SYSTEM tablespace. You can drop the SYSAUX
tablespace only if you have the SYSDBA system privilege and you have
started the database in MIGRATE mode.
From the 10.1 Administrators Guide:
If the SYSAUX tablespace becomes unavailable, core database
functionality will remain operational. The database features that use
the SYSAUX tablespace could fail, or function with limited capability.
http://oradbpedia.com/wiki/The_SYSAUX_Tablespace has this take:
The SYSAUX tablespace is a new tablespace that is required in Oracle
Database 10g...
When you migrate to Oracle Database 10g, the SYSAUX tablespace needs
to be created as a part of that migration. This is done after mounting
the database under the new Oracle Database 10g database software. Once
you have mounted it, open the database in migrate mode with the
startup migrate command. After the database is open, you can create
the SYSAUX tablespace.
Note: The loss of the SYSAUX tablespace is not fatal to your database.
In our testing it appears that the only real impact is that certain
functionality related to the occupants of the SYSAUX tablespace is
lost.
Earlier versions did not have a SYSAUX tablespace:
SYSAUX is the name of the compulsory tablespace, introduced in Oracle
10g.
If all else fails read the documentation. Try here for starters (v. good diagram). In Oracle, a schema is a database. Also see here - particularly this:
One characteristic of an RDBMS is the independence of physical data
storage from logical data structures. In Oracle Database, a database
schema is a collection of logical data structures, or schema objects
[i.e. related tables]. A database schema is owned by a database user
and has the same name as the user name.
A further distinction is between the physical database files "the database" and the "instance" - the running software that makes up the fully working system.
In one sense, an Oracle "database" is useless - it's just a bunch of bits on a disk - it's the software which turns it into a system (i.e. a database in the sense that information is retrievable/modifiable).
An Oracle database is a running instance plus data (+ control + redo &c.) files which itself may contain 1 or many schemas which are themselves "databases" - i.e. as the layman would understand them - i.e. a HR, CRM or ERP (&...) schemas/systems.
Best Answer
Tradition and the ability for elegant failure degradation.
As the database has evolved over 11 versions, important views and tables that maintain and support the database have been programmed into the two tablespaces. Furthermore, they represent a fantastic logical separation between the "NEVER EVER TOUCH" and the "third parties can put stuff here."
From the docs:
Thus, while it's critical to have the
SYSTEMtablespace never ever fail, one doesn't need to be nearly as paranoid for SYSAUX, this allows the designers to keep the size of SYSTEM down, while allowing third-party auxiliary features to be part of the "core" database.