The ‘Select All Categories and Taxonomies‘ plugin is the simplest way
to limit users and authors to a specific category.
Users can add several categories to a post by checking the category’s box by default. The default category checkboxes are replaced with radio buttons by this WordPress plugin.
First, you’ll need to install and activate the Select All Categories and Taxonomies plugin.
After activation, navigate to Settings » Moove taxonomy buttons.
Then, in the ‘Category’ dropdown, select ‘Radio button.’
