Which tool you will use to join some cells and place the content at the middle…

Concept: Joining several cells into one AND centering the content inside it is exactly what a "Merge and Center" control does — it performs the merge and the centering in a single action. This is different from a plain "merge" action (which only joins the cells, without touching alignment) and from a plain "center" action (which only changes alignment, without joining any cells).

Application: In Excel 2003 and earlier, this combined merge-and-center action sits on the Formatting toolbar as the Merge and Center tool — clicking it once merges the selected cells into a single cell and centers the content in that cell. (In Excel 2007 and later the same action is the Merge & Center icon on the Home ribbon.)

Contrast: Checking each of the other offered routes against both requirements — join the cells AND center the content:

• Ticking only the Merge cells checkbox in the Format Cells dialog box joins the selected cells into one, but it does not touch alignment — the content keeps whatever alignment it already had.

• Choosing Centered alignment in the Format Cells dialog box changes only how the content sits within each cell — the cells themselves are never combined.

• The Alignment tab of the Format Cells dialog box offers a Merge cells checkbox and, separately, horizontal-alignment choices such as Center Across Selection — it does not contain a single checkbox that performs both the merge and the centering together.

So the only route that satisfies both parts of the question — merging the cells and centering the content — in one step is the toolbar's Merge and Center tool.