Indexing is the correct answer because it creates a data structure that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space to maintain the index data structure. Like a book’s index that allows you to quickly find information without reading the entire book, a database index allows the database to find data without scanning the entire table. Full table scans can be time-consuming and impact performance, especially on large tables, hence are not the best for quick data retrieval. Caching temporarily stores frequently accessed data for quick access, but this is not a direct design element of the database itself; rather, it's a process that may be implemented in conjunction with the database. Sharding is a database architecture pattern that involves splitting a large database into smaller, more manageable pieces, but it doesn't inherently improve data retrieval speed on its own. Vertical scaling increases the capacity of a single server by adding more resources, but it also doesn't directly impact the speed of data retrieval operations.