In general computer knowledge, which of the following is not an operating system but instead a web browser application used to access websites on various platforms?

Introduction / Context:
This general knowledge question asks you to distinguish between operating systems and web browser applications. Operating systems manage hardware resources and provide a platform on which applications run. Web browsers, on the other hand, are applications that run on top of operating systems and are used to access and display content from the internet. Recognizing which names correspond to operating systems and which refer to browsers is important in basic computer literacy.

Given Data / Assumptions:

• The question asks which option is not an operating system.
• The options include Android, Vista, iOS, and Opera.
• We assume that Android, Windows Vista, and iOS are known operating systems from major vendors.
• We also assume that Opera is recognized as a web browser application available on multiple operating systems.

Concept / Approach:
An operating system is system software that manages the computer hardware and provides essential services to application programs. Examples include Android for mobile devices, Windows Vista for desktop and laptop computers, and iOS for Apple mobile devices. Opera is not an operating system; it is a web browser that runs on top of operating systems such as Windows, macOS, Linux, Android, and iOS. Browsers like Opera, Chrome, Firefox, and Edge rely on the underlying operating system for resource management and hardware access. Therefore, among the given options, Opera is the one that is not an operating system.

Step-by-Step Solution:
Step 1: Identify which entries in the list are commonly described as operating systems.Step 2: Recognize that Android is a mobile operating system, Windows Vista is a desktop operating system, and iOS is a mobile operating system from Apple.Step 3: Recall that Opera is widely known as a web browser, similar to Chrome or Firefox.Step 4: Note that Opera does not control hardware resources or provide system level services; it runs as an application.Step 5: Conclude that Opera is not an operating system and therefore is the correct answer to the question.

Verification / Alternative check:
Official product pages for Android, Windows Vista, and iOS describe them as operating systems that manage devices and run apps. Documentation for Opera, in contrast, describes it as a web browser with features such as tabbed browsing, integrated search, and extensions. Users download and install Opera on an existing operating system rather than using it as the system software itself. This clear difference in how the products are described and installed confirms that Opera is not an operating system.

Why Other Options Are Wrong:
Option A is incorrect because Android is a widely used mobile operating system, especially on smartphones and tablets. Option B is incorrect because Vista refers to Windows Vista, a full operating system from Microsoft for personal computers. Option C is incorrect because iOS is Apple's mobile operating system that runs on iPhones and iPads. Only option D, Opera, a cross platform web browser application rather than a full operating system, is not an operating system and thus correctly answers the question.

Common Pitfalls:
Some learners confuse brands, thinking that any well known software name might be an operating system. Others may assume that because browsers interact closely with the internet and sometimes with device features, they must be part of the system software. A good strategy is to ask whether the software is responsible for booting the device and managing hardware resources; if not, it is likely an application rather than an operating system. Remembering examples of widely used browsers such as Opera, Chrome, and Firefox helps reinforce this distinction.

Final Answer:
The correct answer is Opera, a cross platform web browser application rather than a full operating system.