Final answer:
During the Late Triassic period, the dominant plants included cycads and ferns. Cycads were gymnosperms with large leaves often mistaken for palms. The correct answer is C. cycads, with ferns also being present at that time.
Step-by-step explanation:
During the Late Triassic period, a variety of plants existed, several of which are mentioned in the question provided. Among them, ferns, cycads, and palms are the candidates presented.
Cycads, in particular, thrived during the Mesozoic era and are known to have been a dominant form of plant life during that time. Cycads are gymnosperms with large, finely-divided leaves, often mistaken for palms because of their appearance, and grew as short shrubs and trees in tropical regions.
Ferns are also noted for their abundance in prehistoric times, particularly before the Cretaceous-Paleogene boundary, and evidence shows a variety of fern spores found in strata from different geological periods, including the Late Triassic.
Therefore, the correct answer to the question 'Late Triassic trees included ___________.' would be C. cycads, and to a lesser extent, B. ferns, as both were present during that time period. Palms, on the other hand, are not gymnosperms and their fossil record does not date back to the Triassic. The correct answer is C.