Final answer:
Herodotus's book 'The Histories' is a primary source as it provides direct accounts from the time of the Greco-Persian Wars.
Step-by-step explanation:
The book called The Histories written by the ancient Greek historian Herodotus around 440 BCE is considered a primary source. This is because it is a document that comes directly from the time period to which it refers, offering firsthand accounts and insight into the Greco-Persian Wars. As Herodotus traveled and based his work on primary sources, his own writings serve as evidence for historians looking into this period of history. Herodotus's approach to history-writing reflects the practice of explaining human motivations and events of the time, which set the standard for modern historiography.