Comprehensive and Detailed In-Depth Explanation:

UC wants to restrict the "Answer Questions with Knowledge" action to a subset of Knowledge articles. Let’s evaluate the options for scoping agent access.

Option A: Define scope instructions to the agent specifying a list of allowed article titles or IDs.Agent instructions in Agent Builder guide behavior but cannot enforce granular data access restrictions like a specific list of article titles or IDs. This approach is impractical and bypasses Salesforce’s security model, making it incorrect.

Option B: Update the Data Library Retriever to filter on a custom field on the Knowledge article.While Data Library Retrievers in Data Cloud can filter data, this requires custom development (e.g., modifying indexing logic) and assumes articles are ingested with a custom field for filtering. This is less straightforward than native Knowledge features and not a standard option, making it incorrect.

Option C: Assign Data Categories to Knowledge articles, and define Data Category filters in the Agentforce Data Library.Salesforce Knowledge uses Data Categories to organize articles (e.g., by topic or type). In Agentforce, when configuring a Data Library with Knowledge, you can apply Data Category filters to limit which articles the agent accesses. For the "Answer Questions with Knowledge" action, this ensures the agent only retrieves articles within the specified categories, aligning with UC’s goal. This is a native, documented solution, making it the correct answer.

Why Option C is Correct:

Using Data Categories and filters in the Data Library is the recommended, scalable way to limit Knowledge article access for agent actions, as per Salesforce documentation.