Final answer:
The earliest vascular plants were likely monilophytes, which include ferns and their relatives. Bryophytes are non-vascular and were among the first land plants, while gymnosperms and angiosperms appeared later in the evolutionary timeline.
Step-by-step explanation:
The question 'Earliest vascular plants are?' pertains to the history of plant evolution, specifically the rise of vascular plants. Vascular plants first evolved approximately 420 million years ago, likely from bryophyte-like ancestors. These early vascular plants featured a life cycle dominated by the diploid sporophyte generation. The correct answer to the student's question is E. monilophytes, which are a group that includes ferns and their relatives, and are some of the earliest vascular plants.
Bryophytes, which include mosses, liverworts, and hornworts, are the closest living relatives to the first terrestrial plants, but they are non-vascular. Gymnosperms and angiosperms evolved later, with gymnosperms cycling back to the Carboniferous period and angiosperms dominating from the mid-Cretaceous period onward.