Final answer:
The least important reason why Pepys's Diary is significant to historians is its appeal to sensory details, as it is regarded as less crucial compared to the diary being a first-hand account, recording historical events, and depicting a specific era.
Step-by-step explanation:
The least important reason why Pepys's Diary is significant to historians is likely option C, as it appeals to sensory details. While the appeal to sensory details does provide a vivid picture of the era, which is beneficial, it is considered less crucial for the historicity of the document compared to the other options listed. When analyzing primary sources like diaries, historians value first-hand accounts that record important historical events and paint a picture of a specific time and place because they offer direct insights into the past and can serve to understand the historical context.
Primary sources like Pepys's Diary are valuable because they were written during the time being studied and offer an unfiltered perspective. Although the sensory details can enrich the narrative and draw readers into the historical context, it is not the driving force behind a historian's interest in such a document.