robfelty

Category reminder uses javascript to assert that a category has been assigned 
to a post before publishing it. If no category is assigned, then it will 
prompt the user to assign before publishing.

Category Reminder

This plugin requires users to assign a category to each post other than "Uncategorized".

longbsvnu

Quick Search And Only One Category Posting support quickly selecting category via search box when there are many categories. Besides, you can use radio button instead of checkbox when you want clients to choose 1 category only. 
Quick Search And Only One Category Posting supports all custom post types and custom taxonomy activated in theme and plugin.

Quick Search Only One Category Posting

Quick Search And Only One Category Posting support quickly selecting category via search box when there are many categories.

Category Reminder vs Quick Search Only One Category Posting