The correct option is System message.

A System message is the chat role that you use to provide high level instructions that govern what the model should and should not do and it is also where you define precise reply structure by giving templates examples or a required schema. Putting output format rules and prohibitions in a System message signals that these are authoritative instructions that apply throughout the conversation.

Grounding context provides external facts or documents to make responses more accurate but it does not by itself enforce behavioral rules or a strict response format so it does not match this description.

Few shot learning uses example input and output pairs to teach a pattern through examples but examples rarely serve as explicit prohibitions or guaranteed structural templates the way a System message does.

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