The minimum age prescribed in India for its citizens to cast their vote is 18 years. This means that individuals who are 18 years or older are eligible to participate in the democratic process by choosing their representatives. Option 2, 21 years, is not the correct answer as it exceeds the actual minimum age requirement. Option 3, 16 years, is also incorrect as it is below the minimum age requirement. Option 4, 20 years, is also incorrect as it is not the minimum age prescribed for voting in India. Therefore, the correct answer is option 1, 18 years. It is important for citizens to exercise their right to vote as it is a fundamental aspect of democracy and allows individuals to have a say in the governance of the country.