The correct answer is D. a and b only because a prompt is the input provided by a user to a generative AI model. In natural language systems such as ChatGPT and other language models, the prompt is usually written as text in natural language. It may be a question, instruction, command, description, context, example, or task requirement that guides the model toward producing a response.

Statement A is correct because prompts are the user-provided input that generative models use to produce outputs. Statement B is also correct because, for ChatGPT and similar models, prompts commonly appear as natural language text. Statement C is not fully correct because prompts are an important way to guide model output, but they are not the only possible control mechanism. Outputs can also be influenced by system instructions, model settings, retrieval context, fine-tuning, guardrails, and application design. Therefore, the best answer is D. a and b only.