What is the common characteristic of hard candies?

Hard candies are primarily characterized by their high sugar concentration and solid form. This crystallization process results in a firm texture that remains stable at room temperature, allowing for a variety of flavors and colors to be incorporated. The sugar concentration is crucial as it not only defines the candy's firmness but also contributes to its sweet taste and helps to preserve the candy by inhibiting microbial growth.

In contrast, other options, such as those suggesting high milk content or a soft, chewy texture, are characteristic of different types of confections. For instance, creamy or soft candies typically include ingredients like milk or fats, which contribute to a different mouthfeel and texture, distinguishing them from hard candies.