D. By aggressively suppressing nationalist movements.
During the 19th and early 20th centuries, powerful European states responded to the growth of nationalism by aggressively suppressing nationalist movements. Nationalism threatened the power and stability of these states, as it was often associated with demands for greater autonomy, independence, and self-determination for various ethnic, linguistic, and cultural groups.
European powers used a range of tactics to suppress nationalist movements, including censorship, propaganda, imprisonment, and violence. They often sought to portray nationalists as unpatriotic or disloyal, and to divide and conquer different nationalist groups by pitting them against each other.
Some European states also attempted to co-opt nationalist movements by offering limited concessions or reforms, such as cultural rights or limited autonomy, in order to defuse tensions and maintain their dominance. However, these efforts were often too little, too late, and failed to address the root causes of nationalist grievances.
Overall, the response of powerful European states to the growth of nationalism during the 19th and early 20th centuries was characterized by repression and resistance rather than accommodation or reconciliation.