Final answer:
A novel where a speaking pigeon warns of the Mount Vesuvius eruption would be classified as 3. modern fantasy due to the inclusion of a fantastical element within a historical setting.
Step-by-step explanation:
A novel set in ancient Pompeii where a pigeon saves a family from the eruption of Mount Vesuvius by whispering a warning into a child's ear would be classified as modern fantasy. This genre is characterized by plot-based writing where the narrative includes elements that are beyond the realm of the current reality, often featuring magical or supernatural events.
The backdrop of Pompeii and the historic eruption of Mount Vesuvius gives it a strong connection to real events, but the introduction of a speaking pigeon pushing the narrative into fantasy. Historical fiction, by contrast, would maintain plausible scenarios within the historical setting. Therefore, the whispering pigeon, acting as a magical or fantastical element, clearly defines the novel as modern fantasy.