Final answer:
An aphorism is the correct answer as it is a short, memorable statement of principle, differing from metonymy, parallelism, and ethos which are distinct types of rhetorical devices.
Step-by-step explanation:
The rhetorical device that can be defined as a short, memorable statement of principle is D) Aphorism. This is because an aphorism is a concise statement designed to make a point or illustrate a commonly held belief. The other options listed, such as Metonymy, Parallelism, and Ethos, are different types of rhetorical devices.
Metonymy is a figure of speech in which one thing is represented by another that is commonly and often physically associated with it. Parallelism is a device where parts of the sentence are grammatically the same, or are similar in construction. It can be a word, a phrase, or an entire sentence repeated. Ethos is a rhetorical appeal to an audience based on the speaker's credibility.