고생대(Gosaengdae) (= during the Paleozoic era.)
Explanation&Learning Korean
This question is a play on words in Korean. The term “고생대” (Gosaengdae) refers to the Paleozoic era in geological terms. However, in Korean, “고생” also means “suffering” or “hardship,” and “대” can mean “period.” So, “고생대” sounds like “the era of suffering,” which humorously implies that it was a time when the Earth was ‘tired’ or ‘had a hard time.’ This kind of pun is a common feature in Korean nonsensical quizzes, where the humor often arises from homophones or similar-sounding words.