A. Specify the public and private keys as the options under Administration > Platform > Options.
B. No action is needed; encrypted flat files are automatically detected and decrypted by Adobe Campaign.
C. No action is needed. Encrypted data should be stored as encrypted.
D. Specify a pre-processing decryption command in the Data Loading (file)activity.
Explanation:
Reference: https://docs.adobe.com/content/help/en/campaign-classic/using/automating-with-workflows/general-operation/importing-data.html