To do Alt + A U C on Mac in Excel, download and install Accelerator Keys, then press Option, followed by A, U, C.
Command: Clear Outline
This is how to do Alt + A U C on Mac in Excel with Accelerator Keys, the native ribbon and alt key shortcut solution for Mac users who get more done with Excel.
Alt + A U C clear Outline removes all outline groupings from the worksheet. This flattens the data structure while keeping all rows and columns visible.