The visibility of solution articles can be set by entering the necessary solution articles in the "Solutions" tab under the Portal categories of the respective portal. Please navigate to Admin ­> Channels > Portals > Corresponding portal name and add these articles in its solutions tab. 

If you have articles common to more than one or two portals, kindly click on edit in the category to choose the portals the category must be visible in. 

Further, visibility could be set to logged-in users or all users within the folder where it could be changed according to your requirement. Another alternative is to set user permission for the solutions in Admin -> Channels -> Portals -> settings -> User Permissions -> who can view the solution articles.