SQL is primarily used to query data contained in which of the following structures?

SQL, or Structured Query Language, is specifically designed for managing and manipulating data stored within relational database management systems (RDBMS). This makes databases the primary structures for which SQL is intended. A database consists of tables that hold data, and SQL allows users to perform various operations on this data, such as querying, inserting, updating, and deleting records, as well as managing database schema.

While applications might use SQL to interact with data, they do not store data themselves in a way that SQL can directly query. Similarly, files, such as plain text files or CSV files, are not structured databases and typically require different methods or languages (like Python or other file-handling methods) for data manipulation. Spreadsheets, while they can display data similarly to databases, are not considered databases in the traditional sense and do not fully support SQL. Therefore, the structure most closely aligned with SQL's purpose and capability is a database.