Final answer:
Cassandra creates a partition index for each SSTable, enabling quick data location. This is different from the tree structures and hash index options provided.
Step-by-step explanation:
For each SSTable, Cassandra creates a partition index. The correct answer is not explicitly listed among the options provided (a B-tree, LSM-tree, hash, or skip list). Cassandra uses a partition index which is somewhat related to a sparse index. It helps in quickly locating the data without having to scan the entire SSTable. It stores the index at regular intervals within the SSTable thereby allowing efficient searches for data. Cassandra's storage engine is designed around an LSM-tree (Log-Structured Merge-tree) structure, however, this refers to the overall architecture of the storage engine rather than the specific type of index used for SSTables.