Dense index is a type of secondary index in a database that stores a key-value pair for every record in the database. In a dense index, the index file contains an entry for every record, making it a comprehensive index that allows for efficient searching and retrieval of data.

Primary Index:
- A primary index is a type of index that is based on the primary key of a table.
- It provides a one-to-one mapping between the records in the table and the index entries.
- It is typically implemented as a B-tree structure, where each node in the tree corresponds to a range of values in the primary key.
- Primary index is not a dense index because it does not contain an entry for every record in the database.

Clustered Index:
- A clustered index is a type of index that determines the physical order of data in a table.
- It is implemented by reordering the data in the table to match the order of the index.
- The leaf nodes of a clustered index contain the actual data pages, so there is no need for a separate index file.
- Clustered index is not a dense index because it does not contain an entry for every record in the database.

Secondary Index:
- A secondary index is an index that is based on a non-key attribute of a table.
- It provides an additional path to access data based on a different attribute other than the primary key.
- A secondary index can be either dense or sparse.
- A dense secondary index contains an index entry for every record in the database, making it a comprehensive index.
- Hence, the correct answer is option 'C' - Secondary index.

Primary Non-Key Index:
- A primary non-key index is not a commonly used term in database indexing.
- It seems to refer to an index that is based on a non-key attribute and is specifically designed for primary access.
- Without further context or clarification, it is difficult to determine the exact meaning and characteristics of a primary non-key index.