An index is clustered, if
Index can be created using any column or combination of column which need not be unique. So A,B are not the ans.
Indexed column is used to sort rows of table.Whole data record of file is sorted using index so C is correct option. (Simple video explains this).
Clustered Index is created on data file whose file recordrds are physically ordered on a non key field which does not have distinct value for each record .
Option A ) Candidate Key
Option B) Primary Key
Option D ) File NOT ordered
Best choice is Option C
Option C ) Data records are ordered