The $(Si_2O_5)_n^{2n-}$ anion is obtained when:

• A
  no oxygen of a $SiO_4^{4-}$ tetrahedron is shared with another $SiO_4^{4-}$ tetrahedron
• B
  one oxygen of a $SiO_4^{4-}$ tetrahedron is shared with another $SiO_4^{4-}$ tetrahedron
• C
  two oxygen of a $SiO_4^{4-}$ tetrahedron are shared with another $SiO_4^{4-}$ tetrahedron
• D
  three oxygen of a $SiO_4^{4-}$ tetrahedron are shared with another $SiO_4^{4-}$ tetrahedron