Explanation:
Vacuum flotation is a wastewater treatment process that involves the attachment of large impurities to air bubbles to remove them from the wastewater. This process is commonly used in the treatment of industrial wastewater and is effective in removing solids and oils.

Vacuum Flotation Process:
1. Introduction:
- Vacuum flotation is a variation of the conventional flotation process.
- It involves the introduction of air bubbles into the wastewater under reduced pressure conditions.

2. Creation of Vacuum:
- A vacuum is created in the flotation tank using a vacuum pump.
- The reduced pressure helps to generate small air bubbles in the wastewater.

3. Attachment of Air Bubbles:
- The air bubbles rise to the surface of the wastewater due to buoyancy.
- As the bubbles rise, they attach themselves to the large impurities present in the wastewater.
- The impurities adhere to the air bubbles and form a froth layer on the surface.

4. Separation of Impurities:
- The froth layer containing the attached impurities is skimmed off from the surface of the wastewater.
- This separation process effectively removes the large impurities from the wastewater.

5. Advantages of Vacuum Flotation:
- Vacuum flotation offers several advantages over other wastewater treatment processes.
- It can effectively remove large impurities, such as solids and oils, from the wastewater.
- The process is relatively simple and does not require extensive equipment or chemicals.
- It can be used as a pre-treatment step before further treatment processes, such as biological treatment or disinfection.

In the given options, vacuum flotation (option C) is the correct answer as it specifically involves the attachment of large impurities to air bubbles to remove them from the wastewater. The other options do not involve this specific mechanism for removing impurities from wastewater. Sludge digestion tank (option A) is a process for the decomposition of organic matter in sludge, primary sedimentation (option B) is a process for the separation of suspended solids from wastewater, and oxidation pond (option D) is a treatment process that uses natural processes to remove impurities from wastewater.