In computer science, what is meant by the concept of distributed systems, and how does it differ from a single centralized system?

Introduction / Context:
Distributed systems have become the foundation of modern computing, from cloud services to large scale web applications. Instead of one central computer serving all requests, many networked machines cooperate to provide a unified service. This question asks you to recognise the core idea behind distributed systems and how they are perceived by users.

Given Data / Assumptions:

• Multiple computers are connected over a network.
• These computers coordinate their actions by passing messages.
• To the user, the system often appears as a single integrated service.
• The focus is on logical behaviour, not on physical location.

Concept / Approach:
A distributed system consists of independent computers that do not share memory or a single clock but communicate over a network. Each node runs its own operating system and can fail independently. Software layers on top of this hardware aim to hide the distribution, so that users see a single coherent system. Examples include distributed file systems, clusters, grid computing platforms and cloud infrastructure. The key idea is both distribution and cooperation, which distinguishes distributed systems from isolated machines or monolithic mainframes.

Step-by-Step Solution:
Step 1: Identify that the question refers to systems composed of multiple computers, not just one. Step 2: Recall that in a distributed system, these computers work together over a network to provide services. Step 3: Note that an important design goal is transparency, so that users experience the system as if it were a single computer. Step 4: Compare this with centralised systems, where a single machine hosts all resources and services. Step 5: Choose the option that explicitly mentions independent computers that cooperate and appear as one coherent system.

Verification / Alternative check:
Standard definitions in distributed systems literature describe them as collections of autonomous computers connected by a network and equipped with distributed system software. Authors emphasise that the system should look like a single coherent system from the perspective of users and applications. This directly supports the description given in the correct option and rules out options that involve only one machine or unrelated applications.

Why Other Options Are Wrong:
Option B describes a centralised mainframe system rather than a distributed one. Option C talks about a stand alone personal computer, which is explicitly not distributed. Option D describes multiple applications on a single machine with no cooperation or networking, which does not fit the definition of a distributed system where independent nodes collaborate over a network.

Common Pitfalls:
Learners sometimes think that any networked computers automatically form a distributed system, but casual file sharing does not always involve the level of integration associated with distributed system design. Another pitfall is to focus only on physical distribution and forget the important goal of making the system look unified and coherent to its users despite being spread across many machines.

Final Answer:
A distributed system is a collection of independent computers that cooperate over a network to share resources and services and that appear to users as a single coherent system.