IMO, another valid answer to this question is "Clustered OR nonclustered index, depending on the specification of other constraints". Tying in with the building a unique constraint on the FK column, then it would be as defined. I can also see how this could be interpreted as it would build an index, regardless of other constraints, and in that case it would be wrong.
Wayne
Microsoft Certified Master: SQL Server 2008
Author - SQL Server T-SQL Recipes