A family of Microsoft word processing software products for creating web, email, and print documents.
The option is only available if you access the Options dialog box when a document is open (but I'm sure you already know that).
What happens if you start Word in Safe mode? Then is the option available?
Here's how to start in Safe mode: Exit Word if it is currently running. Hold the Windows logo key as you press R; in the Run dialog box, type winword /safe and press Enter.