The correct answer is D. Both what to say and when to say something.
A natural language generation program must decide both what to say and when to say something. The program must first understand the context of the situation, including the user’s intent, the information that is available, and the constraints on the output. The program then must generate text that is relevant, accurate, and understandable. The program must also decide when to say something, taking into account the user’s attention and the flow of conversation.
Option A is incorrect because the program must also decide when to say something. Option B is incorrect because the program must also decide what to say. Option C is incorrect because the program does not need to know why it is being used in order to generate text.