In order to achieve the desired a custom filtering behavior should be created in order to differentiate the groups and the items (a part of the group). The theme of the AutoComplete control needs also to be modified in order to apply different styles for the group and the item. The could be achieved by using implicit styles. Please, check the following article from our help documentation that provides a more detailed information about how to create a custom filtering: http://docs.telerik.com/devtools/wpf/controls/radautocompletebox/features/filteringbehavior
We have created a sample project that demonstrates how to achieve the desired. Please, give it a try and let us know if it worked for you.
Hopes this helps.
Regards,
Nasko
Telerik
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