The letters A and B are a source of research that may bias, so these options would not be appropriate for a historian to use as a source for his or her unbiased research.
The letter C does not have a source classified as source material as it is the result of a survey using other source materials.
The letter D presents source material that presents valid, impartial and accurate information about the Second World War. For this reason, this would be the best source of research for the historian to use.
