Your question is Secure Local Data Storage. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Explain how you implement secure data storage and encryption locally on a mobile device. Cover Android and iOS storage primitives, key generation and protection, encryption modes, backup handling, access control, and behavior when the device is rooted, jailbroken, lost, or offline. Explain how you would verify the implementation through code review, automated tests, and runtime security testing.