20) C

Remember that a chessboard already has two colours: black and white.

If the circle is contained within a square, it could be the opposite colour of the square and no rules will be broken.

However, if the circle was larger than a square and not contained within a square, it would have to be different colour to black and white as no same colour can be both sides of the border. Hence, either none or one extra colour would always be needed.

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