An effective plot line that moves the story forward should include which element? a. characters who are secretive or mysterious. b. foreshadowing that hints at what's coming next. c. a conflict that is unresolved at the end of the story. d. a device that forces unnatural action into the plot.

It's probably D. A device that forces unnatural action in the plot. But it could quite possibly be B. (I'm so helpful aren't I?) 
All the others are not plot, they hint at plot.

Here's my reasoning.

For instance; A lady owns a flower shop and says a lot of mysterious things about her flower shop that may or may not allude to something. That is not plot, or at least plot, plot.
This is plot; A lady who owns a flower shop has a secret lair for gangs and crooks under her building, and she is their leader, her main goal in life is to own all the flowers in the world and she goes about it by murdering people.
 
(Terrible analogy, but you get the picture.) Show your characters actually doing something or being interrupted by something and not just have them say something mysterious like, "He is coming for you." But then never explaining who, what or where that person is or came from.

The point being; actually have a story line and not characters that sit around all day sipping tea and saying, "Oh hum. I wish something would actually happen in this book." That's the key to writing plot. Make something happen. Hope that helps! 

If you look at the surrounding words, you can get a general basis of what another word for inviolate would be. i would say that it is pristine