In Android, how does an application run in isolation and remain sandboxed from other applications that are installed on the same device?

Introduction / Context:
Security and isolation are key design goals of the Android platform. This question examines how Android ensures that an application runs in its own sandbox and cannot freely access data or code from other applications. Understanding this model is essential for designing secure apps and reasoning about permissions and interprocess communication.

Given Data / Assumptions:

• Multiple applications are installed on a single Android device.
• Each application should have its own private space for code and data.
• The operating system must prevent unauthorized access between applications.

Concept / Approach:
Android implements sandboxing primarily through the Linux kernel. Each application runs in its own Linux process and is assigned a unique Linux user ID. File permissions and process boundaries enforce isolation. In addition, each application normally runs in its own instance of the Android Runtime, which further separates memory and execution context. When applications need to interact, they use well defined mechanisms such as Intents, bound Services, and content providers, all controlled by permissions.

Step-by-Step Solution:
Step 1: Recognize that Linux process isolation and user ID separation are fundamental operating system features.Step 2: Understand that Android assigns a distinct Linux user ID to each application at install time.Step 3: Realize that file access on the device is restricted by these user IDs, so one application cannot normally read another application private files.Step 4: Note that each application typically runs in its own virtual machine instance, which separates memory space and prevents accidental interference.Step 5: This behavior is captured in option A, which describes per app processes, unique user IDs, and sandbox enforcement.

Verification / Alternative check:
Android security documentation emphasizes that application sandboxing relies on Linux user based protection and process isolation. Only applications that share the same signing certificate and explicitly request a shared user ID can run in the same process. Most apps are isolated by default. Reviewing these facts validates that option A accurately reflects how isolation is implemented.

Why Other Options Are Wrong:
Option B suggests that all applications share one process, which would eliminate isolation and contradict the design. Option C focuses only on encrypting Activity classes, which is not how Android enforces sandboxing; encryption does not replace user ID based protection. Option D is the opposite of reality, since forcing a shared user ID would remove the per app security boundary and is not done by default.

Common Pitfalls:
Some developers mistakenly assume that running code in the same process or using shared user IDs is harmless, without realizing the security implications. Others underestimate the role of permissions and content provider access control in protecting data. A clear understanding of the sandbox model helps developers decide when and how to share data securely using explicit APIs while preserving isolation by default.

Final Answer:
By running each application in its own Linux process with a unique user ID and separate virtual machine, enforcing sandboxing and permission checks.