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"
"Index Tuning Recommendations," http://msdn.microsoft.com/library/default.asp?url=/library/en-us/optimsql/odp_tun_1_6583.asp
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.