The correct answer is C because the agent must retrieve from more than one grounding source: company-specific PDFs and a comprehensive Knowledge base. An ensemble retriever is appropriate when multiple retrievers or multiple indexed sources must be combined into one retrieval strategy. That lets the Service Agent search across both unstructured document content and Knowledge content without forcing the specialist to choose only one source. An individual retriever is designed for a single retrieval target, so it is too narrow for this requirement. “Dynamic retriever” is not the best match for the stated need because the issue is not runtime source selection; it is combining multiple authoritative sources. Salesforce guidance describes custom retrievers as supporting more data source patterns, including ensemble retrievers, for Agentforce Data Library use cases.