What is responsible for the breakdown of organic matter in sewage?

The correct answer is "microbes." 

Microbes, such as bacteria and fungi, are responsible for breaking down organic matter in sewage. When organic matter enters a sewage treatment plant, microbes use it as a food source and break it down into simpler compounds. This process is essential to sewage treatment because it helps to reduce the biological oxygen demand (BOD) of the wastewater. 

The BOD is a measure of the amount of oxygen required by the microbes to break down the organic matter in the water. If the microbes are removed from the sewage treatment, the organic wastes will not be broken down, and the BOD will not reduce, leading to the accumulation of wastes in the treatment plant. Air, water, and sand do not play a significant role in the breakdown of organic matter in sewage.