Which of the following is not a constituent for making porcelain insulators?

The correct answer is D. Silica.

Porcelain insulators are made from a mixture of kaolin, quartz, feldspar, and water. Kaolin is a clay mineral that provides the insulator with its strength and durability. Quartz is a hard mineral that provides the insulator with its electrical resistance. Feldspar is a mineral that provides the insulator with its thermal stability. Water is used to bind the other ingredients together.

Silica is a mineral that is found in many different types of rocks and minerals. It is the main component of quartz. Silica is also found in sand, glass, and concrete. However, silica is not a constituent for making porcelain insulators.