The correct answer is option 1: Bryophytes. Bryophytes are often referred to as the "amphibians of the plant kingdom" because they share characteristics with both algae and land plants.

Option 2: Thallophytes refers to a larger group of plants that includes algae and fungi. Thallophytes are not specifically known as the "amphibians of the plant kingdom."

Option 3: Pteridophytes are a group of plants that include ferns and their relatives. While pteridophytes have their own unique characteristics, they are not typically referred to as "amphibians of the plant kingdom."

Option 4: Gymnosperms are plants that produce naked seeds, such as conifers. They are not categorized as "amphibians of the plant kingdom."

In summary, the correct answer is Bryophytes (option 1), which are often called the "amphibians of the plant kingdom" due to their similarities to both algae and land plants.