The statement that best describes a defining feature of a compound adjective is:
It is usually a made-up word.
However, this is somewhat misleading. The more accurate understanding is that a compound adjective typically consists of more than one word and is often hyphenated when used before a noun (e.g., "well-known author").
If the options presented do not include "It consists of more than one word" as a defining feature, then "It is always hyphenated" would be the best choice, though it's worth noting that not all compound adjectives are hyphenated in every context.
Given the options, if you have to choose one, "It consists of more than one word" is the clearest defining feature.
Is there a particular answer structure you'd like to follow when answering?