Finder tag search exclusive
Open a Finder window at the folder in which you wish to search.  Press ⌘F to open a Spotlight search window, which will target that folder.  There is a drop-down list that will have "Kind" selected:



You will want to change this to "Raw Query", which, if it isn't available in the drop-down list, you can add by selecting "Other..." at the bottom of the list, then selecting it in the panel of search attributes that is presented:



As your raw query, enter the following noting the triple equality:

kMDItemUserTags==='fruits'


Then click "Save" to create your Smart Folder.



Here's a demonstration using the coloured tags in Finder on the contents of the following folder:



This finds files tagged only Green (triple equality):



This finds files tagged Green ± other tags (double equality):