When raw water contains little or no noncarbonate hardness, it can normally be softened through the use of?

The correct choice is based on the understanding of water chemistry and the principles of water softening. When raw water has little or no noncarbonate hardness, it primarily contains calcium and magnesium in the form of carbonates. Lime, which is calcium hydroxide, effectively addresses this type of hardness by reacting with the carbonic acid present in the water. The reaction leads to precipitation, reducing the hardness of the water.

Adding lime alone is sufficient in this scenario because it forms insoluble calcium carbonate when it reacts with the bicarbonates in the water, effectively softening it. This process also raises the pH, which contributes to further removing hardness.

The alternatives offered, such as using equal parts lime and soda ash, soda ash alone, or caustic soda, introduce additional complexities or are suited for water with different hardness profiles. These methods are often utilized when noncarbonate hardness is present or when a higher degree of softening is required, but they are not necessary when the raw water is primarily alkaline due to carbonates. Therefore, lime alone is the most straightforward and effective method in this specific case.