Final answer:
To duplicate a sparkline in an Excel worksheet across multiple cells, the Autofill feature is the most efficient. Once the initial sparkline is created, use the fill handle to drag and copy it down the Trend column.
Step-by-step explanation:
To copy the sparkline into all the vacant cells of the Trend column in the Average Call Time worksheet, the Excel feature you would use is Autofill. This feature allows you to extend a series or copy adjacent data into a selected range of cells, which is ideal for duplication of sparklines. You would first create a sparkline in the first cell of the Trend column. Then, you select that cell, position your cursor at the bottom right corner of the cell until it turns into a plus sign (the fill handle), click and drag down through all the cells where you want to copy the sparkline.
It is important to note that Copy-Paste could also be used, but it would be less efficient for a large number of cells. Sparkline Painter is not an existing Excel feature, and Data Validation is used for controlling the type of data entered into a cell, not for copying data or cell formatting.