The answer is D) what to wear.
Parallelism is a theme, because she used "where to go, when to leave, and..." instead of "where she would go, when she would leave, and..." then the answer is "what to wear".
Let's put the sentence together, "where to go, when to leave, and what to wear". THAT is a parallel sentence!
Now let's try what most would think the answer is which is, C) "what she should wear"... "Where to go, when to leave, and what she should wear", THIS is a normal sentence since the beginning is not the same as the end of the sentence.
That's why it's called parallelism, the beginning is the same as the end. It <em>Mirrors </em>the beginning.
Now if the sentence was, "where she would go, when she would leave, and..." then C would work.
"Where she would go, when she would leave, and what she should wear."
I hoped I helped! Everyone have a blessed day! <3