Answer: An ancient record on payments for the building of a pyramid
Explanation: If the historian wanted to know more about the life of the labourers in ancient Egypt, the most trusted source would be a record on payments for the building of a pyramid. In this way, besides how much the payment was, much more could be learned, what was the attitude towards the workers in terms of pay, their position in society, how well they could live or not, etc.
All other options offered regarding potential historical sources can give us details about religion, art, warfare, customs, such as a fiction book of a tomb or a letter from Pharaoh advising his son to become a good warrior, etc.