Complete the student's dramatic email to their history professor.

Professor, I was absolutely horrified ___ that my pet iguana had eaten my entire essay on the French Revolution!

The correct answer is to discover.

Many adjectives that express emotions or reactions (like horrified, happy, surprised, or sad) are followed by a to-infinitive to explain the reason for that feeling. In this case, the student was horrified because they discovered what the iguana did!