Answer: C
Correct answer: XFD1048576
XFD1048576 is the last cell of a modern worksheet: column XFD is the 16,384th and row 1,048,576 is the last. A cell address is always column letters followed by row number, which disqualifies 1A. Rows are numbered from 1, so A0 does not exist. And column letters run A to Z, then AA to ZZ, then AAA to XFD - three letters at most - so AAAA5 is beyond the grid. The lettering is a base-26 system: after Z comes AA, and after AZ comes BA. The 27th column is AA and the 703rd is AAA. Both limits are powers of two, \( 2^{14} \) columns and \( 2^{20} \) rows, raised from 256 and 65,536 when the .xlsx format arrived in Excel 2007. Ctrl + Right Arrow from an empty row travels to column XFD, and Ctrl + Down Arrow to the last row, which is the quickest way to see the boundary.