A principal of design that i see in the image is unity because the room represents the ocean and there are fish surrounding which brings everything together and balances out the message the author is trying to get across. hope that helps :)
Several principles of design are utilized within this image. For example, there are contrasting colors of blue (to enforce an aquarium-like effect) and orange (to emphasize the goldfish). Emphasis is also considered a principle of design so because there is a large emphasis on the goldfish (by both quantity and color), this can be listed as an answer. You can go so far as to argue there is pattern and repetition because of the number of goldfish being included within the photo. Basically, there are several answers to this question, but I believe contrast is the biggest principle in use here.
Possibly A or C? it could also be B though... the best advice I can give you is to eliminate 1 or 2 of the choices then try to find the answer from there! uhm,, hope this helps :)
