Which feature is common to all ferns?

Ferns are unique among plant groups because they reproduce through spores instead of seeds, which is a defining characteristic of their life cycle. The reproductive structures of ferns, called sporangia, produce spores that disperse into the environment and can develop into a new plant under suitable conditions. This is a fundamental trait shared by all ferns, distinguishing them from seed-producing plants such as flowering plants and gymnosperms, which use seeds for reproduction.

The other features listed are not applicable to all ferns. For instance, ferns do not produce seeds or flowers—these characteristics are specific to flowering plants and some gymnosperms. Additionally, ferns are classified as vascular plants because they possess specialized tissues (xylem and phloem) for the transport of water and nutrients, which contrasts with non-vascular plants like mosses or liverworts that do not have these systems. Therefore, reproducing through spores is indeed the common feature among all ferns.