If you find that there's no Design tab in Microsoft Word, verify if the Design tab is enabled in Word Options under Customize Ribbon.

Click the "Developer" tab that now appears in the Ribbon above your Word page. Click "Design Mode" in the Controls group to turn on Design Mode. This allows you to make changes to the drop-down box.

Microsoft Word’s content controls make it easier than ever to use dependent controls in your templates to update content all at once.

It’s a lot easier to create custom forms in Word than it used to be. This article covers six Content Controls, pre-programmed tools for adding and customizing interactive content in your Word ...

In the Controls group, click Design mode. Doing so will change the appearance of the check box controls (Figure C). Select the one for Mary and then click Properties in the Controls group.