What feature would you use to automatically fill a series of data in Excel?

The Fill Handle is the correct choice for automatically filling a series of data in Excel. This feature allows users to quickly replicate or extend a sequence of numbers, dates, or other data types simply by clicking and dragging a small square located at the bottom-right corner of the selected cell or range of cells.

When you use the Fill Handle, Excel intelligently recognizes patterns based on the initial data you input. For example, if you start with the numbers 1 and 2 in consecutive cells, then dragging the Fill Handle will automatically extend this pattern, filling the next cells with 3, 4, 5, and so on. This significantly speeds up the data entry process, especially for long sequences or repeating patterns.

Other choices such as Data Series and AutoComplete serve different purposes. Data Series allows for a more guided approach to generating sequences, but the Fill Handle provides more flexibility and ease of use for quick filling. AutoComplete helps with text entries but does not function as a filling tool for series. Data Validation is primarily used for ensuring data integrity and does not relate to filling data series. Thus, the Fill Handle is the most effective tool for this task in Excel.