The correct answer is: D. No error
The sentence is grammatically correct. The word “this” refers to the event of Troy being taken by the Greeks, and the word “which” refers to the story of Troy. The sentence is clear and easy to understand.
The sentence could be improved by adding more detail. For example, the sentence could say “Troy was taken by the Greeks after a ten-year siege. This formed the basis of a story that has become famous.” This would provide more information about the event and the story.
The sentence could also be improved by using more specific language. For
example, the sentence could say “Troy was taken by the Greeks after a ten-year siege. This formed the basis of the story of the Trojan War.” This would make it clear that the story is about the Trojan War.