I am also of the opinion that the answer is incorrect.

The question specifically asks for extensions (subclauses) to the GROUP BY clause.

GROUPING_ID is a function, NOT a sub-clause of the...

There are several references to the fact that the meaning of the 'B' in B-Tree is undefined, but since this site is 'Microsoft' SQL Server related, the following page makes...

So why does the documentation for the restorehistory table show a setting of 'V' = Verifyonly for column restore_type?

http://msdn.microsoft.com/en-us/library/ms187408.aspx