Solved by: Legend
In this challenge an andorid APK was given.
Initially I installed the apk on android vritual device to check what’s the app is about and to know what’s happening with the app. But there was nothing informative.
Then with the help of apktool I decompiled the apk to see what’s the functioning of the app and also to look for flag string. Their were lot’s of sub-directories so I juse used grep to see if the flag might be present in plain text, and got the flag.