The correct answer is: D. both (a) and (b)
A natural language generation program must decide what to say and when to say something. It must also consider the context in which it is being used, such as the audience, the purpose of the communication, and the tone of voice that is appropriate.
Option A is correct because a natural language generation program must decide what to say. It must select the words and phrases that will convey the intended message in a clear and concise way.
Option B is also correct because a natural language generation program must decide when to say something. It must choose the right time to speak, taking into account the current situation and the needs of the audience.
Option C is incorrect because a natural language generation program does not need to decide why it is being used. This is a decision that is made by the person or organization that is using the program.
Option E is incorrect because both options A and B are correct.