What is the characteristic of hard water?

Hard water is characterized by its high mineral content, primarily calcium and magnesium ions. When soap is used in hard water, these minerals react with the soap, forming insoluble compounds known as soap scum instead of allowing the soap to lather effectively. This is why hard water does not easily form a lather with soap.

In contrast, soft water contains fewer minerals and thus allows soap to dissolve and lather readily. The presence of minerals is essential to the definition of hard water, thereby ruling out options that suggest it doesn't contain minerals or that it is completely soft. Thus, the characteristic nature of hard water is best described by its difficulty in forming lather with soap.