An opinion may not appear in the final composed stage because USD resolves competing opinions according to composition strength. NVIDIA's Learn OpenUSD material explains that a layer stack is the ordered set of a layer and its recursively gathered sublayers, with the root layer considered strongest, followed by sublayers according to their composed order. It also states that strength ordering determines which opinion is composed into the final stage when multiple layer stacks or layers contribute data for the same prim or property. (docs.nvidia.com)

Option D is correct because a weaker layer can author a valid opinion, but that opinion can be hidden by a stronger authored opinion on the same target. Option A is incorrect because sublayers do not need to be referenced by every other layer to affect a stage. Option B is incorrect because there is no general ''weaker keyword'' that lowers priority. Option C is incorrect because opinions from sublayers, references, payloads, variants, inherits, and specializes can all contribute to composition. This aligns with Composition Sublayers, Layer Stacks, Opinion Strength, and LIVERPS Strength Ordering.