Final answer:
The best summary of Russia's experience in WWI's Eastern front is that Russian troops won several battles but incurred heavy losses due to supply shortages, poor infrastructure, and the resulting low morale which also contributed to the fall of the Tsarist regime.
Step-by-step explanation:
The correct statement that best summarizes Russia's experience on World War I's Eastern front is C) Russian troops won several battles but suffered extremely heavy casualties. Despite early successes, such as the Brusilov Offensive against Austrian forces, the Russian Army was plagued by severe supply shortages, inadequate industrial infrastructure, and poor logistics, which resulted in heavy losses. The Battle of Tannenberg serves as a stark example of these deficiencies, where Germans surrounded Russian armies and inflicted devastating casualties. Moreover, internal turmoil significantly affected morale, leading to desertions and contributing to the eventual downfall of the Tsarist regime and the rise of the Bolsheviks.