The correct answer is option 3: Cold deserts. Tundras are characterized by extremely cold temperatures and a lack of trees. They are found in regions near the Arctic Circle, such as parts of Alaska, Canada, and Russia. Tundras have a short growing season and a layer of permafrost (permanent frozen ground) beneath the surface. Due to these harsh conditions, trees cannot grow in tundras, earning them the classification of cold deserts. While deciduous forests (option 1) have trees that lose their leaves in the winter and are found in temperate regions, tundras are not known for having trees. Tropical rainforests (option 2) are found in warm and wet regions near the equator, quite contrasting the cold temperatures of tundras. Therefore, the correct option is 3: Cold deserts.