Answer:
The diary of someone who lived in the time and place of the story would be the most useful source material for a writer of historical fiction to develop a character, as it can provide insights into daily life, attitudes, beliefs, and experiences of people during that time period. Secondary sources interpreting data from the era of the story can also be helpful, but they may not provide the same level of detail and personal perspective as primary sources like diaries, letters, and autobiographies. Encyclopedias and statistical information may provide background information but are less likely to help in character development.