A- 4 : Thickening of sludge is done by separation of water by floatation or gravity.

1. By floatation : The objective of thickening of sludge by floatation is to involve or attach very small air bubbles to suspended solids and cause the solids to separate from the water in an upward direction. These suspended solids have a lower specific gravity than the air bubbles, which helps to make them float.

2. By gravity : Gravity process of thickening involves the concentration of thin sludge to more dense sludge in special circular tanks designed for this purpose. Tanks consist of slow-moving vertical paddles. Sludge flows into the tank and it removes excess water from the solids collecting at the bottom.

B - 3: Stabilization of sludge is done by digestion of sludge. These are two types of digestion :

1. Aerobic digestion : It is a process of treating the secondary sludge from the biological wastewater treatment process. This process is also known as aerobic stabilization. In this, microorganisms extend into a respiration profile where materials previously stored are oxidized, resulting in a reduction of organic matter.

2. Anaerobic digestion : It is one of the oldest process used for sludge stabilization. This process involves the decomposition of organic and inorganic matter in the absence of oxygen. Many organic effluents and waste comes from industrial, municipal activities are treated with anaerobic digestion.

C - 2 : Conditioning of sludge is done by separation of water by heat or chemical treatment.

1. Heat treatment : It involves heating of sludge for a short period of time under pressure 1 to 1.4  The temperature should be 140 to 2000C  Heat treatment coagulates the solids, breaks down the gel structure and reduces the water affinity of sludge solids.

2.Chemical treatment : It is a common process of conditioning of sludge, which results in coagulation of solids and release the water absorbed. The chemical used in this treatment include ferric chloride, lime, alum and organic polymers

 D - 1: Reduction of sludge is done by decrease in volume of sludge by chemical oxidation process. This process involves a pre-treatment technique to breakdown the component of sludge and make them suitable for biological degradation. Oxidants involve in this process are hydrogen peroxide, chlorine gas, ozone, oxygen etc.

Hence, the correct option is (C).