How to respond when dont know answer at interview

How to respond when dont know answer at interview
Problem
1) I don’t know the anything about the question.
2) I am stuck some where. I don’t know how to proceed.
3) I have made some assumptions but don’t know if they are correct. I don’t know whether the current approach is correct or not. What do I do?
Incorrect Response
A) I don’t know the solution.
B) I tried it long enough, I am unable to proceed ahead.
C) I know a solution which is not optimal, I should not tell that solution and sitting silently.
D) This is my solution (but not stating the assumptions made).
Correct Response
i) As I clarified earlier I am not aware of this particular domain to answer this question. I am aware of these basics only, that too I learned them while I am reading some technical blogs in my free time. So I am not confident of attempting this question because I will end up doing some guess work. If you think this question does’t need the in depth domain knowledge can you explain me the question again so that i will try.