If you don't have alot of units under your belt, a C can hurt your gpa alot. If yuo have alot of units done already, a C probably won't hurt your gpa. I personally would withdraw rather than get a C, because mainly its very time consuming to retake it later for such a low gpa boost if you decide to take the C.
Calculate your gpa w/ the C and without the C, and make a decision based on that.
P.S. stop getting C's, if you get too many C's, it will be extremely difficult to make up for them in the long run.