You can't make this a guessing game as to whether the correct answer is "essen" or "fressen." The tips section tells us "fressen" is for animals and is only used to insult someone if used for a human. In every other exercise with an animal eating "fressen" is the verb used. It is marked wrong if "essen" is used. Why is this different?
You have to memorize it. There are a lot of random ones it seems, but this has helped me, especially if I had to make an educated guess: https://docs.google.com/spreadsheets/d/1ak8kffdi9qbhIRN5m6XcqYVtNMSiATSdXfA0Ceq2GgA/edit?usp=sharing
It is a Google Spreadsheet