What makes Thucydides' job as a scholar of history more challenging? A. Act of romantic elements
B. Biases of eyewitness observers
C. His desire to please immediate public
D. His failing memory

The most obvious struggle was that he was born around 460 BC, meaning information was difficult to get and store. Another big part of that difficulty was that his works lacked the myth and exaggeration that many political and philosophical texts included in that time. so i think it's D, his failing memory!!