The correct answer is: A. Anaerobic bacteria.
Septic tanks are designed to decompose organic waste in the absence of oxygen. This is done by anaerobic bacteria, which are bacteria that can live and grow in the absence of oxygen. Anaerobic bacteria break down organic waste into simpler compounds, such as carbon dioxide, methane, and water. These compounds are then released into the environment through the septic tank drain field.
Aerobic bacteria, on the other hand, require oxygen to live and grow. They are not able to decompose organic waste in the absence of oxygen. Therefore, they are not found in septic tanks.
Both types of bacteria are important in the decomposition of organic waste. However, anaerobic bacteria are the primary decomposers in septic tanks.