I believe the answer is: a. The mirror cracks from one side to the other.
Although Lancelot rode by on his horse which caused her to look it was the fact that she looked directly at Camelot that caused the curse to take effect. The mirror cracking is what symbolizes the curse has taken effect and it happened right after she looked down at Camelot, which if i remember correctly was the one thing she wasn't allowed to do.