All the options provided can help readers understand words, but if we consider which option specifically aids in defining words, "etymological patterns" would be particularly helpful. Etymology reveals the origin and historical development of words, which can provide context and deeper understanding of their meanings.
However, if the goal is to define a word in a more general sense, "parts of speech" can also be important, as knowing whether a word is a noun, verb, etc., contributes to its definition. So, the strongest response directly related to defining words could be "etymological patterns."
If you're asked to select just one, I'd recommend "etymological patterns."