The correct option is B -12
Cyclic silicate : If two oxygen atoms per tetrahedron are shared to form closed rings. The structure with general formula (SiO2−3)n or (SiO3)2n−n
Comparing cyclic silicate anion [Si6O18]n− with (SiO3)2n−n, n=6.
thus overall charge on cyclic silicate anion = −2n=−2×6=−12.
Hence correct option is b.