The correct answer is C. a custom Microsoft 365 Copilot agent . Microsoft Learn explains that Agent Builder in Microsoft 365 Copilot lets you create agents with specific instructions , dedicated knowledge sources , and starter prompts . Starter prompts are designed to help users understand the most common supported scenarios, which directly matches the requirement to provide users with a list of common queries.
Microsoft also documents that an agent can be grounded in selected SharePoint sites, folders, or files , allowing the response scope to be targeted to the HR policy content in Site1 rather than broad enterprise or web data.
The other options do not fit the requirement. Copilot in Word is document-focused and is not intended to create a reusable, shared query experience grounded only in one SharePoint source. Copilot notebooks group materials and chats, but they are not the right tool for publishing a guided HR policy assistant with starter prompts. Researcher is designed for broader, multi-step research using work data and web content, so it does not satisfy the requirement to keep answers grounded only in Site1.