The correctly punctuated sentence is A. 'I did not know the right answer: a or b?'
They are marks, spacings, and signs which help in understanding the correct reading of a written text used to separate sentences and clarify meanings.
The sentence in option A gave emphasis to the statement made and gave a clearer and clarified meaning to better understand the correct reading of the text which stated the confusion in knowing the right answer.
A colon is used to give emphasis, present dialogue, announce or introduce lists and draw attention to something.
Hence, to use a colon, it has to follow an independent clause (that is, a clause that can stand alone as a sentence) and can be used to present an explanation, draw attention or join ideas.
Therefore, the correct answer is option A.
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