This is the 12652nd most frequent Korean word.
대중교통을
"Public transportation."
대중교통을 is used as the object of the verb 이용하면 (if you use), showing the usage of public transportation for environmental purposes.
대중교통을 이용하면 환경 보호에 도움이 됩니다.
Using public transportation helps protect the environment.
대중교통을 is once again used as the object of the verb 선호하는 (preferring), highlighting preferences for public transport.
많은 사람이 대중교통을 선호하는 이유는 편리하기 때문입니다.
The reason many people prefer public transportation is because it is convenient.
대중교통을 is employed as the object of the verb 탈 때 (when riding), focusing on the action of riding public transportation.