What does parallel structure do for sentences? it makes them more interesting and difficult to understand. it makes them more interesting and easy to understand. it makes them more complex and difficult to understand. it makes them more complex and easy to understand.

in june, we will have been married for eighteen months. (i am assuming this is what you want? the sentence, as you have presented it, is grammatically incorrect--   you want it corrected, yes? )

answer is b,

"the tracer looks harmless in flight, but the pilot’s bitten lips and white knuckles suggest how dangerous it can be."

it was correct.