Final answer:
The earliest known flowering plants date to the Cretaceous period of the late Mesozoic era, about 125 million years ago.
Step-by-step explanation:
The earliest known flowering plants, or angiosperms, date to the Cretaceous (late Mesozoic era). Fossil evidence indicates that flowering plants first appeared about 125 million years ago in the Lower Cretaceous and were rapidly diversifying by the Middle Cretaceous, about 100 million years ago.
Prior to the angiosperms, the landscape was dominated by gymnosperms during the Triassic and Jurassic periods of the Mesozoic era. However, by the mid-Cretaceous, a significant number of diverse flowering plants began to appear in the fossil record, marking the beginning of their evolutionary dominance in terrestrial ecosystems.
Therefore, the correct answer to the question is B. Cretaceous (late Mesozoic era).