The diagram below shows the geologic time scale, which separates Earth's history into different eons, eras, periods, and epochs.

The rock layer that the scientist is studying, which contains predominantly large reptiles along with a few small mammals and birds, is most likely from the **Mesozoic Era** (Option B). This era is known as the "Age of Reptiles" and includes the time when dinosaurs were the dominant terrestrial vertebrates.

The presence of very large reptiles suggests that the layer belonged to the Mesozoic, while the smaller reptiles and mammals present in the layer above indicate a transitional period, which is consistent with the mass extinction event that marked the end of the Mesozoic and the beginning of the Cenozoic Era, when mammals became more abundant and diverse.

Therefore, the correct answer is **B. Mesozoic**.