On iOS, why does an app often behave differently when running in the foreground compared with when it is in the background?

Introduction / Context:
Mobile operating systems such as iOS carefully manage background activities to protect battery life, performance, and user experience. When an app moves from the foreground to the background, the system may suspend it, restrict its network access, or allow only certain background modes. As a result, an app can behave very differently depending on whether it is currently visible to the user. This question asks you to identify the main reason for this differing behaviour on iOS.

Given Data / Assumptions:

• We are dealing with iOS application lifecycle, including states such as active, inactive, background, and suspended.
• Foreground apps interact directly with the user and are visible on screen.
• Background apps may be hidden and not directly interacting with the user.
• Battery life and resource management are critical concerns on mobile devices.

Concept / Approach:
On iOS, when an app moves to the background, the system calls lifecycle methods such as applicationDidEnterBackground or sceneDidEnterBackground. After giving the app a short time to save state or finish current tasks, the system may suspend it, meaning it stops executing code but remains in memory. Only apps that declare specific background modes, such as audio playback, VoIP, location updates, or background fetch, are allowed limited ongoing activity. This design reduces energy consumption and keeps the user interface responsive for the app currently in the foreground. Therefore the underlying reason for behavioural differences is that iOS deliberately limits what background apps can do to preserve battery and performance.

Step-by-Step Solution:
Step 1: Recognise that iOS defines distinct app states and transitions controlled by the operating system. Step 2: Understand that when the user leaves an app, iOS often suspends it after a brief grace period. Step 3: Note that only certain background tasks are allowed, and continuous processing is generally not permitted. Step 4: Review option a, which explains that iOS restricts background apps to preserve battery life and performance, suspending or limiting their execution. Step 5: Compare this with other options that make unrealistic claims about hardware shutdown or forced advertisements.

Verification / Alternative check:
Apple developer documentation for iOS clearly describes app lifecycle behaviour, including how apps are suspended in the background and only resumed when brought back to the foreground or when specific background modes are active. It emphasises that this behaviour is designed to extend battery life and maintain system responsiveness. This matches the explanation in option a and does not support the statements in the other options.

Why Other Options Are Wrong:
Option b claims that the central processing unit shuts down whenever the Home button is pressed, which is incorrect; the device continues running and may host many background tasks. Option c suggests that apps in the background change programming language runtimes, which is not how iOS manages execution. Option d states that Apple requires background apps to show full screen video advertisements, which is not a system requirement and would contradict good user experience practices.

Common Pitfalls:
Developers sometimes assume that their code continues running freely when the app is in the background and forget to handle suspension correctly, leading to lost state or missed updates. Another pitfall is not requesting the appropriate background modes when the app genuinely needs limited background activity, resulting in unexpected pauses. For exam answers, remember the general rule: iOS restricts background activity to protect battery life and performance, which explains why behaviour differs between foreground and background states.

Final Answer:
An iOS app behaves differently in the background because the iOS operating system restricts what background apps can do to preserve battery life and performance, suspending or limiting their execution when they are not visible.