Final answer:
Flowering plants first appeared during the Middle Jurassic (A).
Step-by-step explanation:
Flowering plants first appeared during the Middle Jurassic.
Fossil evidence indicates that flowering plants first appeared about 125 million years ago in the Lower Cretaceous (late in the Mesozoic era), and were rapidly diversifying by about 100 million years ago in the Middle Cretaceous. Earlier traces of angiosperms are scarce, but fossilized pollen recovered from Jurassic geological material has been attributed to angiosperms.
Therefore, the correct answer is A. Middle Jurassic.