Option 1: The 42nd Amendment Act is incorrect because it was passed in 1976 and aimed at making several changes to the Constitution to strengthen the position of the Prime Minister and the Parliament. It did not address the voting age.

Option 2: The correct answer. The 61st Amendment Act, passed in 1988, is the amendment that reduced the voting age from 21 years to 18 years. This was done to give the youth a greater voice in the democratic process.

Option 3: The 74th Amendment Act, passed in 1992, is incorrect because it pertains to the constitutional provisions for urban local bodies and does not address the voting age.

Option 4: The 83rd Amendment Act is incorrect. No such amendment exists in the Constitution of India.

Therefore, the correct answer is option 2, the 61st Amendment Act.