Final answer:
A reasonable assumption about a CI that is part of a subsystem is that it can be independently managed and tracked throughout its lifecycle.
Step-by-step explanation:
A reasonable assumption about an item that has been identified as a Configuration Item (CI) and is part of a subsystem is that it can be independently managed and tracked throughout its lifecycle. A CI is a component or resource that is necessary to deliver a service, and it can include physical items like hardware or software, as well as documentation or processes. In the context of a subsystem, the CI would be a specific element within that system that can be configured as part of the overall system design.