Answer: b)A dense primary index
Explanation: Dense primary key is defined as all the search key belonging to the database has a corresponding index record. The record searching becomes easier and quick with this technique .The drawback of this index is requires large space for storage of records. Index persist of the key value and pointer for the record.
Other options are incorrect because loose index works for only some fraction of keys and compact primary index contains the record in compact column manner. Sparse primary index contains record of indexed area and crowded index is over the crowd ratio index .Thus, the correct option is option(b).