D. parallel structure just means that all of your verbs in the comma series line up in regards to tenses, here. in choice D, it's "BE friendly," "INTRODUCE yourself," and "SMILE." these are all present tense verbs.
the rest of the choices don't match--"exercise" and "stretching" are two different verb forms in choice A, "to go to the library" is the odd one out in choice B, ane "to build" doesn't match with the "-ing" verbs in choice C.
I'd say the answer is 'the visible is more dangerous than the invisible', because Duncan never suspected that his faithful Macbeth would be the one to kill him.
I'm sorry, I don't have enough info to answer the question :) Can you explain?
