The atrocity committed by the Soviet Union that specifically targeted Ukrainians is the "Holodomor." The Holodomor was a devastating famine that occurred in Ukraine from 1932 to 1933, resulting from the policies of the Soviet government, including forced collectivization and grain requisition quotas.
While "kulaks" refers to a group of wealthy peasants targeted during the collectivization process, it is not exclusively a Ukrainian issue. "Attacks on Cossacks" and "Red Terror" are broader historical phenomena and not solely focused on Ukrainians.
So the correct response is:
Holodomor