선진국이 · 선진국을
developed country, advanced nation
Example
한국은 짧은 기간에 선진국 대열에 들어섰습니다.
Korea joined the ranks of developed countries in a short time.
Word parts
선 ahead+진 advance+국 country
Literally 'country that has advanced ahead'.
Good to know
후진국 sounds blunt or insulting today; 개발도상국 is the polite term.