Which is the BEST definition of a Configuration Item (CI)?

This is too narrow because CIs are not limited to hardware or software, and registration in an asset database is a property of an asset, not the defining characteristic of a CI.

This describes a CI record or CI attribute set, not the CI itself; the collection of information describes a CI but is not what a CI is.

The ITIL definition of a CI is intentionally broad: it covers any component that needs to be managed to deliver an IT service, including hardware, software, documentation, and people. The phrase 'is, or will be, under the control of Configuration Management' is taken directly from the ITIL glossary and captures both current and planned CIs.

Information recorded by the Service Desk describes an Incident Record, which is a separate ITIL artifact unrelated to Configuration Management.