What are the mandatory/required oracle 11g tablespaces?
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.