What geographical feature covers a large part of Mexico?

Mountains are indeed a significant geographical feature that covers a large part of Mexico. The Sierra Madre mountain ranges, which include both the Sierra Madre Oriental to the east and Sierra Madre Occidental to the west, are prominent in the country. These mountain ranges not only contribute to the diverse topography of Mexico but also play a vital role in the climate and ecology of the region. They influence weather patterns and are home to unique flora and fauna, making them an essential aspect of Mexico's geographical identity.

In contrast, while Mexico does have plains, deserts, and forests, these features do not dominate the landscape to the extent that mountains do. The diversity in elevation provided by the mountains creates a variety of ecosystems and climates across the country, showcasing the geological complexity of Mexico. Thus, mountains are the correct answer due to their extensive coverage and significance in shaping the country's geography.