Where is the menu option for underling trailing spaces

[Rich611]

I have used a menu option before for underscoring trailing spaces but I can't
find it now.

 

[Jay Freedman]

I have used a menu option before for underscoring trailing spaces but
I can't find it now.

Tools > Options > Compatibility, "Draw underline on trailing spaces".

 

[Suzanne S. Barnhill]

But note that if you don't always want to do this, you can force Word to
underline spaces by making the last one a nonbreaking space
(Ctrl+Shift+Spacebar). If this occurs at the end of a line, you'll need to
put a normal space after it.