What is the effect of applying an index on a data set?

Applying an index to a data set primarily enhances the speed of read actions. When an index is created on one or more columns of a database table, it allows the database management system to locate and access the rows more efficiently during query operations. This is particularly beneficial for read-heavy operations where data is frequently retrieved, as it eliminates the need to scan through entire tables.

By indexing, the database can quickly navigate to the relevant portions of data, significantly reducing the time complexity typically associated with query execution. This is especially crucial in large datasets where the difference in read performance with and without indexing can be substantial.

While the other options may seem relevant, they do not accurately reflect the primary purpose and consequence of indexing. For example, write actions may actually slow down because the database needs to update the index whenever a write operation occurs. An index does not inherently reduce the overall size of the database; in fact, it can increase the disk space used due to the additional structures created. Lastly, while indexes might streamline data retrieval for reads, they can complicate it for less straightforward scenarios, thus not serving the purpose outlined in the correct choice.