The correct answer is JSON.

The Key Phrase Extraction API returns structured data in JSON format when called from the Python client. The client library parses the JSON payload into native Python types so developers work with dictionaries and lists rather than raw markup or free text.

The HTML option is incorrect because the service returns machine readable structured data rather than presentation markup. An extraction API provides key phrases in JSON so they are easy to consume programmatically.

The Both C# and Python option is incorrect because it names client languages rather than a response format. The actual response format is JSON regardless of whether a C# SDK or an Python SDK is used.

The Python option is incorrect because the question asks for the response format and not the client language. JSON is the data format that the API returns.

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