Explanation:

In the Australian Open Men's Singles Final of 2020, **Novak Djokovic** defeated Dominic Thiem in a thrilling five-set match with a score of $6-4, 4-6, 2-6, 6-3, 6-4$. This victory marked Djokovic's eighth Australian Open title. Option (A) Roger Federer is a legendary tennis player but did not win the 2020 Australian Open; he reached the semifinals. Option (C) Dominic Thiem was the runner-up in the 2020 tournament. Option (D) Rafael Nadal is another tennis icon who was defeated in the quarterfinals of the 2020 Australian Open. Therefore, Novak Djokovic is the correct answer.