Before using a FHIR server, a client should first check which document to understand server capabilities?

• A. A server's CapabilityStatement
• B. A server's StructureDefinition
• C. A server's ValueSet
• D. A server's Observation

Answer: (D)

In FHIR, the way to understand what a server can do before you interact with it is to look at the server’s CapabilityStatement. This resource describes the server’s actual capabilities: which resources are supported, which REST interactions are available (read, search, create, update, delete), supported search parameters, formats, and any special operations or profiles the server implements. You typically retrieve it from the server’s metadata endpoint, often via a request like GET [base]/metadata, which returns a CapabilityStatement in JSON or XML.

The other options don’t describe how the server operates at a capability level. A StructureDefinition defines constraints and structure for a resource (for profiling data shapes). A ValueSet lists permissible codes from code systems used within resources. An Observation is a specific data resource representing a measured or collected fact, not a description of the server’s capabilities.

So, checking the CapabilityStatement first gives you the authoritative map of what the server can do and how to interact with it.