Citizens of the Russian Federation undergo military service: only upon contraction, upon reaching a certain age only on a voluntary basis (under contract) by conscription and voluntarily (under contract)

The correct answer is: by conscription and voluntarily (under contract).<br /><br />In the Russian Federation, citizens are required to undergo military service either through conscription or voluntarily under a contract. Conscription is mandatory for all citizens, while voluntary military service under a contract is an option for those who wish to serve for a specified period.