Sedimentation on commercial scale occurs in

Answer: Option C

The correct answer is B: Thickeners.
Let's break down why:

Sedimentation is the process where solid particles settle out of a liquid due to gravity.

Thickeners are specifically designed to perform sedimentation on a large, industrial scale.
They are large tanks where solids are allowed to settle to the bottom, forming a concentrated sludge, while the clarified liquid is removed from the top.

Here's why the other options aren't the best fit:

A: Classifiers: Classifiers separate particles based on size and density, often using a combination of sedimentation and hindered settling, but they're more about separation into different fractions than just bulk removal of solids from a liquid like in sedimentation alone.

C: Rotary drum filters: Rotary drum filters use a rotating drum covered with a filter medium to separate solids from liquids. This is a filtration process, not sedimentation.

D: Cyclones: Cyclones use centrifugal force to separate solids from gases or liquids. While they can separate solids from liquids, they are not primarily based on gravity-driven sedimentation. They are more efficient for separating fine particles.

Therefore, thickeners are the primary equipment used for sedimentation on a commercial scale.