What removes trace amounts of dissolved oxygen in the boiler water?

The correct option is the addition of sodium sulfate to the water. Sodium sulfate aids in the prevention of corrosion and also plays a role in scouring dissolved oxygen from boiler water. When sodium sulfate is added, it helps to facilitate chemical reactions that can effectively remove dissolved oxygen, which is critical because dissolved oxygen can lead to pitting and corrosion within the boiler system.

In other contexts, the boiling of water at high pressure can also reduce dissolved gases, including oxygen, through the physical process of reducing solubility at elevated temperatures. However, adding sodium sulfate is a more direct and efficient chemical method to address trace amounts of dissolved oxygen specifically.

Cooling the water and utilizing a water softener do not effectively remove dissolved oxygen. Cooling the water may actually lead to increased solubility of gases, which is counterproductive. While a water softener is beneficial for removing hardness, it does not target dissolved oxygen directly and would not be effective in this context.