I need to know what the most staistically sound calculation of percentages is for the following:

I have a survey question that has multiple answers and the respondent can choose all, none or any combination of answers. My question is how do I calculate the percentages for each answer. Can I take the total for an individual answer option divided by the total number of respondents that answered that question? Or is it more statistically sound to take the total for an individual answer option divided by the total number for all (combined) answer options?

Answer Options
A - 10
B - 9
C - 11

with a total of 15 respondents, should the percentage for Answer A be:
10/15=66.67% or 10/30=33.33%

THANKS for answering this basic question.