ANS:A - diatomaceous earth

The most common filter aid is diatomaceous earth. Here's a brief overview of each option:

1. Diatomaceous earth:
   • Widely used as a filter aid due to its high porosity and ability to form a permeable, stable, and incompressible filter cake.
   • It is composed of the fossilized remains of diatoms, a type of hard-shelled algae.
2. Calcium silicate:
   • Used as a filter aid in some applications, but it is less common than diatomaceous earth.
3. Sodium carbonate:
   • Not typically used as a filter aid. It is more commonly used in chemical processes as a pH regulator or for water softening.
4. Silica gel:
   • Primarily used as a desiccant for moisture absorption and not commonly used as a filter aid.