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

The correct answer is: only upon conscription, upon reaching a certain age.<br /><br />In the Russian Federation, military service is mandatory for citizens who reach a certain age, typically between the ages of 18 and 27. This is known as conscription or mandatory military service. However, there is also an option for individuals to serve in the military on a voluntary basis, known as voluntary military service or military service under contract. This is not mandatory and is chosen by the individual.