Answer C is correct. Sequencing the physical data requires the use of a clustered index. A primary key does not have to have a clustered index assigned to it.
For more information, see the following sources:
Chapter 2, "Creating Database Objects"
SQL Server 2005 Books Online: "SQL Server Database Engine, Designing and Creating Databases, Indexes, Understanding Indexes"
Within MSDN Library, go to MSDN Home, MSDN Library, Servers and Enterprise Development, SQL Server, SQL Server 2000, Database Design, Physical Database Design, Index Tuning Recommendations.