1. The problem asks: What is the fill handle in spreadsheet software used for? 2. The fill handle is a small square at the bottom-right corner of a selected cell or range of cells. 3. It is used to copy formulas or values to adjacent cells quickly by dragging it across the cells. 4. This feature helps to replicate data or formulas without manually typing them in each cell. 5. The other options like formatting cells, sorting data, or inserting rows/columns are different functions in spreadsheet software. 6. Therefore, the correct answer is: To copy formulas or values to adjacent cells.