I often say "I don't know" when in political discussions with right wing wingnuts, mainly to avoid prolonging the discussion. It often does not stop them, as they think it is their duty to educate me. C fits that.
When someone asks me for advice on a situation that will effect them...as what stocks to invest in...I say I don't know, mainly, because my crystal ball is broken. D fits that.
Sometimes when a wingnut makes a very stupid (accusation against latinos, or Jews, or blacks), I say "I don't know about that", which is my way of politely stating that I do not agree. None of the answers fit that.
Perhaps living in Texas, and being an educated liberal, has caused me abnormal defensive mechanisms with avoidance and non confrontation with wingnuts, even if they are elected officials.
For you choices, C is probably what your instructor wants, but frankly, I don't know.
What is the context? It would depend upon the topic being discussed. In certain cases, it may also be "I don't care."
If you don't make any attempt to know later, it might be A.
It could also be C, an attempt to not be involved.
If it is a scientific topic, and you do not have adequate data to make any decision, it may be B. As found at the end of many research articles, "More research is needed."
However, rather than (D) accepting your own ability, it would be accepting your own inability. That is the only one I can omit with any certainty.
However, like bobpursley, I would slightly favor C.