Cooksonia are believed to be the first

While Cooksonia fossils are distributed globally, most type specimens come from Britain, where they were first discovered in 1937. Cooksonia includes the oldest known plant to have a stem with vascular tissue and is thus a transitional form between the primitive non-vascular bryophytes and the vascular plants.

if your own plato the answer is b. the fern transitioned from sporophyte generation to gametophyte generation.