What type of data can Amazon Redshift Spectrum query?

Amazon Redshift Spectrum is designed to extend the querying capabilities of Amazon Redshift, allowing you to run queries on data that is stored in Amazon S3. This capability enables users to analyze vast amounts of data without the need to load it into the Redshift data warehouse directly.

When the data is stored in Amazon S3, Redshift Spectrum can leverage a wide range of data formats, including CSV, Parquet, and JSON, among others. By querying data in S3, users can analyze it alongside data that is already stored within Redshift, providing significant flexibility for data analysis and effectively allowing a hybrid approach to managing data.

The other options do not accurately represent the capabilities of Redshift Spectrum. For example, data stored on-premises or only in relational databases cannot be queried directly by Redshift Spectrum without first moving that data to S3. Cached data also does not meet the requirements, as Redshift Spectrum specifically interacts with data stored in Amazon S3, rather than accessing temporary or cached data. This focus on S3 data storage is what makes option C the correct choice.