Clustered index is dense or sparse?
Clustering index is applied when records are physically ordered on a nonkey field(called clustering field). As clustering field is nonkey so there can be more than one distinct record for a single clustering field value.

The strategy of clustering index is: There is one entry in the clustering index for each distinct value in the clustering field, containing the value, and a pointer to the first block in the data file that has a record with that value for its clustering field.

So clearly it is a sparse index.