Answer : D

Explanation : The numerical values of the letters in each circle add up to 20.

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Answer : S

Explanation : Taking the numerical value of each letter, they follow the sequence of Prime Numbers.

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Answer : N

Explanation : Starting with the A on the left hand side, move around the shape in a clockwise spiral towards the centre. Letters advance through the alphabet in steps of 4, 5 then 6, repeating this pattern all the way around.