TBT"Must" can have more than one meaning when we are talking about the future, but when we are talking about the past, it is used only to indicate the belief that something did/did not happen. So in D, the author is saying "Apparently we did not work all hours on this, because the meeting was cancelled." This doesn't make sense, whereas it makes perfect sense to say that we didn't NEED to work so much, since the meeting did not occur.
Two other notes:
1) When we talk about the present, "must" works similarly: we are saying that something surely IS happening. When we talk about the future, it is different. It means that something is required ("You must not leave the building") or recommended ("You simply must see that new play!").
2) A real GMAT SC question will not be in first person, so we normally won't have someone reasoning about their own behavior. That's part of what creates the funny meaning in D, since surely the author already
knows how long they did or did not work, and does not need to guess about it.