The correct answer is C-The Soviet Union enjoyed strong economic growth in the 1920s.
Indeed, the chart shows that in 1921 all industrial and agricultural output was drastically reduced. This was due to the fact that Russia was in the middle of the Russian Civil War that followed the two consecutive communist revolutions of 1917. Because of the war, the fields were not grown and the industries did not produce anything as most farmers/workers were dead or fighting.
The Red Army finally prevailed over the White army and as Stalin took the power in 1924, he forced the collectivization of agriculture and industrialization of the country in order to prevent an invasion of capitalist Western powers and their allies. By 1928 it is obvious that the agricultural and industrial output levels have recovered, reaching or surpassing the pre-revolutionary levels. This indicates a strong economic growth.