Which button would you use to combine multiple cells into one?

The Merge button is used to combine multiple cells into one single cell within a spreadsheet application. When you select the cells you want to merge and click the Merge button, it consolidates those selections into a larger cell, which can be particularly useful for creating headings or organizing data in a clearer way. Merging cells can help improve the visual layout of your spreadsheet and make specific parts of the data stand out.

In contrast, the Fill handle is a tool for dragging to fill adjacent cells with a sequence of data. The Border button is for adding borders to the cells, enhancing the visual separation between different areas of data. The Undo button is meant for reversing the most recent action taken in the spreadsheet, not for modifying cell dimensions or structure. Thus, the Merge button is the correct choice for combining cells effectively.