The correct answer is option D, that is, redwood tree forests.
Gymnosperms are seed-bearing vascular plants, in which the seeds or ovules are not enveloped in an ovary. In gymnosperms, the pollen is the male reproductive part, and the cone is the female reproductive part. It includes cycads, conifers, and ginkgo.
The redwood trees are examples of gymnosperms and play an essential role in sustaining the equilibrium of carbon dioxide in the atmosphere, like green plants.