how can we apply row level security on apps in qmc?

To get row level security you will have to define an access section in the load script of your application. Then you can make use of Dynamic Data Reduction. Have a look here to find full explanation and examples:

http://help.qlik.com/en-US/sense/November2017/Subsystems/Hub/Content/Scripting/Security/manage-security-with-section-access.htm

You can also use set analysis if you want to make a chsrt or table behave a certain way regardless of filter selected.