The correct answer is B. Identifying Fields should help Agentforce locate the right Knowledge article efficiently, so concise text or text area fields that summarize the article are the strongest fit. Long article bodies are better treated as content fields because they provide the detailed grounding material after the right article has been found. Option A is wrong because using the full main content as an identifying field can dilute search precision and make retrieval noisy. Option C is wrong because a formula-based concatenated key is usually optimized for system logic, not semantic article discovery.
Salesforce's Knowledge grounding guidance distinguishes identifying fields, which help find relevant articles, from content fields, which provide the detailed information used in responses.