About 3 weeks ago, I was doing some research on a related topic and found that this answer is wrong. I had asked Steve to change this question then.

This QotD should be:

Correct Answer: 3

Explanation: Index types Clustered, Non-clustered, Unique, Non-Clustered with Included columns, Indexed Views, Spatial and Filtered indexes have a B-Tree index structure.

Full-text indexes have a token-based functional index with an inverted, stacked, compressed index structure.

XML Indexes (Primary and Secondary) have a B+-Tree index structure.

References:

Clustered Index Structures: http://msdn.microsoft.com/en-us/library/ms177443.aspx

Non-Clustered Index Structures:http://msdn.microsoft.com/en-us/library/ms177484.aspx

XML Primary Indexes:http://msdn.microsoft.com/en-us/library/ms345121(SQL.90).aspx

XML Secondary Indexes:http://msdn.microsoft.com/en-US/library/ms187508(v=SQL.90).aspx

Spatial Indexes: http://msdn.microsoft.com/en-us/library/bb964712.aspx

Full-Text Indexes: http://technet.microsoft.com/en-us/library/cc879306.aspx

Unique, Filtered, Indexed Views and Indexes with included columns: http://msdn.microsoft.com/en-us/library/ms175049(v=SQL.100).aspx

B-Tree index structures: http://en.wikipedia.org/wiki/B_tree_Indexing

B+-Tree index structures: http://en.wikipedia.org/wiki/B%2B_tree

I'm very sorry for the confusion. I'll get with Steve to see what can be done to get this resolved.

Edit: fixed links to work correctly