**Core Concept**
Aerobic digestion of sludge refers to the process of breaking down organic matter in wastewater treatment plants using oxygen. This process involves microorganisms such as bacteria and protozoa that decompose organic matter, producing carbon dioxide, water, and a reduced sludge.

**Why the Correct Answer is Right**
Aerobic digestion of sludge occurs in a controlled environment with a specific balance of oxygen and microorganisms. Activated sludge is a type of aerobic digestion where microorganisms are suspended in water and aerated to facilitate the breakdown of organic matter. This process is also known as activated sludge treatment or activated sludge process. The aeration provides the necessary oxygen for the microorganisms to thrive and break down the organic matter efficiently.

**Why Each Wrong Option is Incorrect**
**Option A:** Trickling filter is a type of wastewater treatment process that uses a filter medium to support the growth of microorganisms, but it is not specifically designed for aerobic digestion of sludge.

**Option B:** Broad irrigation refers to the process of watering or flooding a large area, and it is not related to the treatment of wastewater or sludge.

**Option D:** Septic tank is a type of wastewater treatment system that uses anaerobic digestion to break down organic matter, which is the opposite of aerobic digestion.

**Clinical Pearl / High-Yield Fact**
Activated sludge treatment is an effective method for removing organic matter and suspended solids from wastewater, and it is widely used in municipal and industrial wastewater treatment plants.

**Correct Answer:** C. Activated sludge