Quiz Question, user to skip if wants to

Chip has it right (of course!), but I think you may be able "simulate" the skipped question...

Have on each multiple choice question an answer saying something like "D - Skip This Question - you may skip up to X questions". 

Then set your question values such that they need only correctly answer a set number correctly  (total available  - the X  amount they can skip).    Example set question values at 20 points each and only require 80 points to pass.   Provide 6 questions, allowing them to "skip" 2 of them (by answering the D-choice) and still pass the quiz.

Hope this helps!