Final answer:
In Cassandra, partitioned data is efficiently looked up using the primary key, consisting of a partition key and optional clustering keys, enabling fast data retrieval.
Step-by-step explanation:
Partitioned data in Cassandra can be looked up using a) Primary Key. The primary key in Cassandra consists of a partition key and an optional set of clustering keys. The partition key determines the distribution of data across various nodes in Cassandra's distributed architecture, while the clustering keys determine the sorting within the partition. Using a primary key allows efficient and fast retrieval of data because Cassandra can quickly locate the node containing the data, and then retrieve it using the precise primary key. Secondary indexes (Index) and other methods like Super Columns are less efficient and not recommended for primary lookups.