What is the empirical formula of sheet silicates ?

Question

\[\begin {array} {1 1} (a)\;(Si_2O_5)_n^{2n} & \quad (b)\;(SiO_3)_n^{2n-} \\ (c)\;(SiO_3)^{n-}_{n} & \quad (d)\;(Si_2O_7)_{n}^{3n-} \end {array}\]