Final answer:
The process by which an attacker gains Shannon information about unknown plaintexts or ciphertexts is known as Data inference, which involves the exploration and examination of indirect evidence and the revision of conclusions based on new information.
Step-by-step explanation:
When an attacker gains Shannon information about plaintexts or ciphertexts that were previously unknown, this process is generally known as Data inference. In this context, data inference refers to the method by which indirect information is gleaned and certain conclusions or deductions about the data are made by analyzing the patterns, metadata, or any leakage from the cryptographic system in use.
Inferring information in this manner requires not just observation, but also a deeper exploration and examination of the evidence at hand. The attacker must then reflect upon their initial assumptions and potentially revise their conclusions if new questions arise or if additional information comes to light. Such revisions can significantly alter the initially drawn conclusions about the security of the system.
Thus, the correct answer to categorize the process by which an attacker gains information about unknown plaintexts or ciphertexts would be Data inference (D).