The correct answer is: D. Chlorinated coppers.
Alum, ferric chloride, and ferric sulphate are all commonly used coagulants for treating sewage. Chlorinated coppers, on the other hand, are not typically used for this purpose because they are more expensive and can be more harmful to the environment.
Alum is a naturally occurring mineral that is used as a coagulant because it can cause suspended particles in water to coagulate, or clump together, so that they can be removed. Ferric chloride and ferric sulphate are both salts of iron that are also used as coagulants. They work in a similar way to alum, by causing suspended particles to coagulate.
Chlorinated coppers are a mixture of copper and chlorine that is sometimes used as a coagulant. However, they are more expensive than alum, ferric chloride, and ferric sulphate, and they can be more harmful to the environment. Chlorinated coppers can also be more difficult to use than other coagulants, and they can cause problems with the taste and odor of water.
For these reasons, chlorinated coppers are not typically used for treating sewage.