Which of the following options lists only open source cloud computing platforms or frameworks that organisations can deploy in their own data centers?

Introduction / Context:
Cloud computing is available both as a public service from large providers and as software platforms that organisations can install and run in their own data centers. Open source cloud computing platforms provide the building blocks to create private or hybrid clouds with features similar to public providers. Recognising examples of such platforms helps distinguish between cloud software and cloud services. This question asks you to identify which option contains only open source cloud computing platform projects rather than commercial public clouds or unrelated services.

Given Data / Assumptions:

• Open source cloud platforms are software stacks that can be downloaded, installed, and modified under an open source licence.
• Examples include Infrastructure as a Service frameworks for managing compute, storage, and networking.
• Public cloud brands such as Amazon Web Services are services provided by companies and are not open source software packages.
• Consumer applications like music or file sharing services are not cloud computing platforms in the infrastructure sense.

Concept / Approach:
Several well known open source projects aim to replicate or provide cloud like functionality. OpenStack is a popular example that manages compute, storage, and networking resources for private and public clouds. Eucalyptus is another project designed to create Infrastructure as a Service environments with compatibility for certain Amazon Web Services application programming interfaces. Apache CloudStack is yet another open source platform that orchestrates virtualised resources to deliver Infrastructure as a Service. These differ from commercial cloud providers like Amazon Web Services or Azure, which are services operated by vendors rather than software frameworks deployed by customers. The correct option must therefore list only projects such as OpenStack, Eucalyptus, and Apache CloudStack.

Step-by-Step Solution:
Step 1: Recall examples of open source cloud platform projects: OpenStack, Eucalyptus, and Apache CloudStack. Step 2: Recognise that Amazon Web Services, Microsoft Azure, and Google Cloud Platform are commercial public clouds, not open source software that can be installed on premises. Step 3: Note that Salesforce, Dropbox, and Spotify are software as a service or consumer web applications, not Infrastructure as a Service frameworks. Step 4: Observe that Oracle Cloud, IBM Cloud, and DigitalOcean are also vendor operated cloud services, not open source platforms. Step 5: Conclude that only option a contains a set of open source cloud computing platforms suitable for building private or hybrid clouds.

Verification / Alternative check:
If you search project documentation, you will find that OpenStack, Eucalyptus, and Apache CloudStack provide source code under open source licences and offer installation guides for private data centers. Companies can download and customise these stacks. In contrast, commercial providers such as Amazon Web Services do not offer their proprietary cloud control software as open source; customers access those environments only as remote services. This verifies that option a correctly lists open source cloud platform projects.

Why Other Options Are Wrong:
Option b lists major public cloud providers, which are services rather than open source platforms. Option c names application level services that run on top of cloud infrastructure rather than infrastructure platforms themselves. Option d again refers to commercial cloud offerings and a hosting provider, none of which are distributed as open source frameworks for customers to run in their own data centers.

Common Pitfalls:
A common confusion is to equate any popular cloud brand with a software product. Students sometimes assume that because a service is accessed over the internet, it must be a cloud platform. In exam questions, pay attention to wording such as open source and platform, which point to software you can install, not just services you can subscribe to. Another pitfall is failing to distinguish between infrastructure level platforms and higher level applications that simply use the cloud.

Final Answer:
Some open source cloud computing platforms are OpenStack, Eucalyptus, and Apache CloudStack, which is the combination given in option a.