Hmm.. I would tell them the truth depending on the situation. But also emphasize how you learned from the mistake and will not repeat it again while in optometry school.
What schools are you interviewing at? Out of the three schools I have interviewed at (NOVA, Indiana Unv, and Ferris) only Indiana really drilled me about grades. The other looked at my grades as a whole and never mentioned anything in particular. At IU they pick on your weak spots on purpose to see how you handle these types of situations. They usually can smell a BS answer...lol
Good Luck! 🙂