The answer is B, Primary Source.
Why? A letter written in the past is an actual historical artifact and is a personal account of something the person experienced. A account of something a person actually experienced is a primary source about whatever events are described.
A biased source would be an account that is from a source that has an incentive to distort history.
A secondary source would be a work that describes or analyzes a primary source. For example, an account written about the letter by me or you would be a secondary source.
A Historigraphical source is a history text that is informed by both primary source and secondary sources.