While the primary goal of most Web form designs is to get people through a form as quickly and painlessly as possible, there are situations where slowing people down is advisable. When choosing between primary and secondary actions, visual distinctions are a useful method for helping people make good choices.
Should this distinction be more prominent like the button vs. link in Option A or a bit more subtle like the two different colored buttons in Option C? Option A fared a bit better in time to completion, average number of fixations, and average total length of fixations indicating people completed the form faster but not by much.