Which of these is considered a personal cloud solution?

A personal cloud solution is one that is primarily designed for individual use, allowing users to store, manage, and access their data from anywhere with an internet connection. Dropbox fits this definition perfectly, as it is primarily a file hosting service that allows users to store their files in the cloud, share them easily with others, and synchronize files across multiple devices.

The other options represent different types of cloud services that are not primarily aimed at individual users. Salesforce is a robust customer relationship management (CRM) platform designed for businesses to manage their customer interactions and data. Google Cloud Platform and Microsoft Azure are examples of infrastructure and platform as a service (IaaS/PaaS) providers meant to support businesses and developers in deploying applications and managing large-scale compute and storage resources. These services typically cater to organizational needs rather than personal use.