The correct answers is - c. orchids.
During the Mesozoic era there were plants in abundance, but most of those plants that dominated then were much different than the plants that dominate nowadays, though some are still present.
The most dominant plants in the Mesozoic were all plants that had seeds, but were not flowering plants, with the conifers being the most common ones, as well as the cycads and the gingkoes.
The orchids, the most diverse flowering family nowadays, evolved during the late Jurassic period, so it was present at the end of the Mesozoic era, but it was not a common plant by any means. The orchids started to become a common plant during the following Cenozoic era.