Nothing is a certainty until it happens and the only option that will work in your sentence is "might".
If you don't study, you might fail the exam.
In the context you have provided, "must" will not work because it means you are obliged/required to fail. But since you have not taken the exam yet, you are not required to fail. Who knows, you may become inspired and get all the questions correct.
The probability of failure is high but its certainty cannot be validated until after the exam. So, it's "might".