Final answer:
The Start_num element specifies which character in the cell range will be the beginning of the extraction.
Step-by-step explanation:
The MID function in Excel is used to extract a specific number of characters from a cell or range of cells. In this case, the element that specifies which character in the cell range will be the beginning of the extraction is the Start_num. This argument indicates the position of the first character to extract from the cell.