Which one of the following anions is present in chain structure silicates?

In chain silicates, two oxygen atoms per $[\text{SiO}_4]^{4-}$ tetrahedron are shared with adjacent tetrahedra to form a linear chain. This sharing results in the general empirical formula $(\text{SiO}_3)_n^{2n-}$, which can also be represented as $(\text{Si}_2\text{O}_6)_n^{4n-}$.

• $[\text{SiO}_4]^{4-}$ is the basic unit of orthosilicates (no oxygen atoms shared).
• $[\text{Si}_2\text{O}_7]^{6-}$ is present in pyrosilicates (one oxygen atom shared).
• $(\text{Si}_2\text{O}_5^{2-})_n$ is present in sheet or phyllosilicates (three oxygen atoms shared).