What defines smart materials?

Smart materials are characterized by their ability to respond dynamically to external stimuli, such as temperature, light, moisture, and stress. This means they can change their properties or behavior in response to changes in their environment, making them incredibly versatile and useful in a variety of applications. For example, some smart materials can change shape or stiffness when exposed to heat or electric current, which allows for innovative designs in fields ranging from aerospace to biomedical engineering.

The other options focus on specific attributes that do not capture the essence of what makes a material "smart." High electrical conductivity, thermal expansion resistance, and maintaining a constant shape describe specific physical properties but do not necessarily imply that the material can respond actively to changes in its surroundings, which is the hallmark of smart materials. Thus, the defining characteristic of smart materials is their responsiveness to external stimuli, which is why that choice is the most accurate.