Consider the following HelloWorld.

The Python snippet authors a reference opinion directly on the prim /refSphere in a new anonymous root layer. Usd.Stage.CreateInMemory() creates an empty stage, and OverridePrim("/refSphere") authors an over prim spec because no concrete type is being defined. The call refSphere.GetReferences().AddReference("./HelloWorld.usda") adds a reference composition arc to that same prim, so the root layer serializes the opinion as prepend references = @./HelloWorld.usda@.

Option C is correct because ExportToString() prints the authored contents of the root layer, not the fully expanded composed result of the referenced asset. The referenced HelloWorld.usda layer has defaultPrim = "hello", so omitting a prim path in AddReference() targets the referenced layer’s default prim. NVIDIA’s reference documentation explains that a reference may omit the prim path when the referenced layer has a default prim, and the target prim will be resolved from that metadata. ( docs.nvidia.com )

Option A is incorrect because the reference is not authored under /hello; /hello exists only inside the referenced file. Option B is incomplete because it omits the reference metadata. This aligns with Composition → References, Default Prim, Layer Authoring, and Exported Layer Opinions .