Lots Of Confused Guys here.
First of All Clustering Index is done on a Field Which is NON KEY & ORDERED!
Because it is Non Key , there can be DUPLICATES, so we can’t have an Index Entry for the 1st element of the Block,we need to have an INDEX entry for each element. (For Duplicates , only one is enough).
So if Main File has DUPLICATES,then Clustered Index is also Sparse Index , bcoz we are not having index entries for the Duplicates.
Now Answer can’t be sparse Index because if we have an INDEX Entry for every record in main File(assuming No Duplicates) , it is NOT a Sparse Index.
IN SHORT : Question says “data records is the same as or close to the ordering of data entries in some index”.
Now these data records can have duplicates ot it might not have.
If there are Duplicates, then it can’t be Dense index, else not.
If there are No duplicates , then it can’t be Sparse Index.
So only Clustering Index can be the answer.