Fill in the blanks with the correct modal
This is impossible, it ______be a mistake!

must

The correct modal to fill in the blank is must.

The modal must is used to express certainty about something that is very likely to be true. In this case, the speaker is certain that the mistake must have been made because it is impossible for the statement to be true otherwise.

The other options are incorrect.

• Ought to is used to express obligation or necessity, but it is not used to express certainty.
• Might is used to express possibility, but it does not convey the same sense of certainty as must.
• Should is used to express advice or a suggestion, but it is not used to express certainty.