If you were composing a piece in c minor, you would need 3 flats in the key signature. some of the key signature are D major has a key signature of F♯ and C♯.
What is key signature?
In musical notation, a key signature is the addition of bright or flat markings on specified lines of varying of a musical column to suggest that the adjacent notes in each octave are to be boosted or lowered uniformly from their natural pitches.
Thus, option A is correct.
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