The number of diagonals of a polygon with ‘ n ’ sides is 
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depending on n (n - 3 )
2 n
i.e. depending on n2 - 3n
2 n
i.e. depending on n2
5 n
i.e. depending on n
5
Hence for a quadrilateral ( n = 4 ) i.e. n < 5 the number of diagonals is less than the number of sides, for a pentagon ( i.e. n = 5 ) the number of diagonal is equal to the number of sides and for ALL other polygons ( with n > 5 ) i.e. from hexagon onwards, the number of sides. Hence can’t say.
Hence D.