Help the detective finish her official report about a rather embarrassing bank heist.

The robbers wouldn't have escaped so easily if the security guard ________ asleep on the job.

The correct answer is hadn't fallen.

In a third conditional sentence, the condition (the if-clause) requires the past perfect tense. "Hadn't fallen" correctly sets up the imaginary past situation. A common mistake is putting "would have" in the if-clause, but it belongs only in the result clause!