Delete Dropdown Option in Numbers

Judging from your screenshot that may not be Pop-Up Menu.

Instead you may have 'Show suggestions when editing table cells' selected at Numbers > Settings.

If so, then the suggestions shown in your screenshot are automatically generated from the values in the cells in the same column (both above and below the cell you are editing).

To fix the problem go through the column and find the value that you don't want. Edit it to the correct value.

Then you won't get the "wrong" suggestion any more.

You can, of course, format the cells in the column as Pop-Up Menu to more carefully control the values that you input. But the automatic suggestions are a handy feature, often all that you need, and they require no work to set up.

SG

Select Format in the toolbar, then Cell, and eliminate the choice you don't want. But a pop-up Menu with only one choice is pretty useless, maybe you'd want to set the format to Automatic or to Checkbox (you need to erase the content of the cell first).

Thank you for your quick reply! I will take your advice on the setting.