Answer: C. He established English as a language worth writing in.
Step-by-step explanation:
After Geoffrey Chaucer had written The Canterbury Tales, English replaced French in elementary schools. This major changed was followed by the parliament's recognition and adoption of the English language as well.
Chaucer managed to prove that writing in English could be as beautiful as writing in other languages. His writings were also relevant to the creation of English language dictionaries and it was thanks to him that the rhyme royal (scheme of a-b-a-b-b-c-c) was introduced in English literature.