Final answer:
A 'walled garden' in mobile security is a model where devices can only access applications from a trusted source, offering a safe environment by ensuring apps are security vetted.
Step-by-step explanation:
The term "walled garden" in the context of mobile security refers to a security model where the device can only download or install applications from a trusted source, such as the official app store of the mobile operating system. This approach is intended to protect users and the enterprise by ensuring that all available apps have been vetted for security issues. Hence, in this context, 'walled garden' relates to option D, which is using a trusted source for application downloads and installations.