Answer:
The correct answer to the question: Which verb will make the sentence express simultaneous events?, would actually be D: Had been writing.
Explanation:
The reason has to do with the difference between the tenses offered as choices here. When you use the simple past, and even the past perfect, you are talking about events that have already passed, and they do not offer the possibility of thinking about any other events that might have taken place during that timeline. The one grammatical structure that offers that chance, and that allows a reader to think about alternate, or simultaneous events, aside from the main one being mentioned, is the past perfect continous, because it is the one tense that speaks about an event, or events, that happen in the past and continue towards the present and even future.
