Which chemical acts as a bacterial nutrient for stabilizing potable water?

Polyphosphate is the correct choice because it serves as a source of phosphorus, which is an essential nutrient for bacterial growth. In the context of stabilizing potable water, maintaining a balanced microbial population is crucial for the treatment process, as certain bacteria play a significant role in breaking down organic material and maintaining overall water quality.

By providing an adequate nutrient supply, polyphosphate helps support the growth of beneficial bacteria, which can ultimately aid in the biological stabilization of water. This process enhances water treatment efficiency and improves the water's overall safety for consumption.

Calcium carbonate, while important in water treatment for pH stabilization and hardness control, does not specifically serve as a nutrient for bacteria. Sodium hypochlorite is primarily used for disinfection and would not support bacteria growth. Alum is a coagulant used to remove impurities from water but does not act as a bacterial nutrient.