Final answer:
The responsibility of Australian citizens aged 18 and over is to participate in voting in federal elections, as it is a civic duty and mandatory under Australian law.
Step-by-step explanation:
Among the responsibilities of Australian citizens who are 18 years or over, the correct answer is a. Voting in federal elections. It's a civic duty in Australia, as in many other democracies, for citizens of legal voting age to participate in the electoral process. Compulsory voting has long been part of Australian law, ensuring that the political system reflects the will of the electorate. While obtaining a driver's license, attending school, and joining the military can be important aspects of life, they are not mandated civic responsibilities like voting.