Fill in the blanks with the correct modal
It _____ what you said about his sister or perhaps he was just nervous.

might have been

The correct modal to fill in the blank is might have been. The modal might have been is used to express possibility about something that happened in the past, and it is the most appropriate modal to use in this context because it conveys the idea that there is a possibility that the person was upset about what you said about his sister, but it is also possible that he was just nervous.

The other options are incorrect. The modal must have been is used to express certainty about something that happened in the past, but there is not enough information to be certain that the person was upset about what you said. The modal should have been is used to express obligation about something that happened in the past, but it is not used to express possibility. The modal ought to be is similar to should have been, but it is more formal.