The answer is D: The Diary of Anne Frank.
The Diary of Anne Frank was written by Anne herself during a period of two years as she was hiding with her family (of Jewish ancestry) from German Nazis during the invasion of the Netherlands during WWII.
This diary tells Anne´s own perspective upon her life and the horrible events that forced her and her family into hiding. Anne would eventually die in a concentration camp. This book is then, clearly, the most important source for any research regarding Anne Frank´s life.