The correct answer is option-1, fermentation. Bacterial decomposition refers to the breakdown of biological material by bacteria. Anaerobic conditions mean that there is no oxygen present. In this scenario, bacteria perform fermentation to break down the organic matter. Fermentation is a process in which bacteria convert sugars into simpler compounds, such as alcohol or lactic acid, without the use of oxygen. This process releases energy for the bacteria to survive. Fertilization (option-2) is the process of adding fertilizers to the soil to enrich its nutrient content. Contamination (option-3) refers to the presence of harmful substances or organisms in an environment. Composting (option-4) involves the natural decomposition of organic material by aerobic bacteria, which requires the presence of oxygen.