What specifies where particular extensions are required or expected?

• A. a resource profile
• B. a CapabilityStatement
• C. StructureDefinition
• D. CodeSystem

Answer: (A)

In FHIR, a resource profile is the way you tailor a resource for a specific use, and it explicitly encodes constraints on where data should appear. When you create a profile for a resource, you can define which extensions are allowed or required in particular elements, set whether they must be present (MustSupport) or can be omitted, and specify their cardinality. This is exactly how you indicate where certain extensions are expected to occur in a real-world implementation.

CapabilityStatement describes what a server can do, not how resources are structured or which extensions must appear. StructureDefinition is the general blueprint used to describe a resource or datatype, but the precise requirements about extensions for a given use case are captured by the resource profile applied to that use case. CodeSystem deals with coded values, not extension placement.