(d)
Orthosilicic acid $\left( H_{4}SiO_{4} \right)$, on heating at high temperature, loses two water molecules and gives silica $(SiO_{2})$ which on reduction with carbon gives carborundum $(SiC)$ and $CO$.
$H_{4}SiO_{4}\underset{- 2H_{2}O}{\overset{\ \ 1000℃\ \ }{\rightarrow}}SiO_{2}\underset{\mathrm{\Delta}}{\overset{\ \ \ C\ \ \ }{\rightarrow}}SiC + CO$
carborundum