The easiest word to begin with is the second blank. The writing is “illuminating” for the adventurous reader. Of the words in blank 2, the word closest in meaning to “illuminating” is edifying. Now, look at blank 1. If these detours “frustrate” some readers, then they are exasperating. Now, look at blank 3. The frustrated readers do not like detours, and are used to “direct” argumentation. Of the choices, the word closest in meaning to “direct” is unequivocal.