The fossilized teeth of ancestors of the modern horse have grown progressively larger and flatter over the past 60 million years. Which of the following is the best explanation for the changes seen in horse tooth structure over time?

Because nowadays, many horses are domesticated, and food they would have eaten in the wild, they don't usually eat anymore. They have learnt to adapt to eating other foods, like hay, that require molars to ingest. This is why over time, their tooth structure has changed.