Magneto-rheostatic fluids are defined by their change in what property?

Magneto-rheostatic fluids are specifically designed to change their viscosity in response to applied magnetic fields. When a magnetic field is introduced, the structure of these fluids alters at a microscopic level, leading to a significant change in their flow characteristics. This property makes them particularly useful in various applications, such as dampers and clutches, where control over fluid movement and resistance is required.

The other options describe different phenomena not associated with magneto-rheostatic fluids. For instance, temperature changes due to magnetic exposure do not relate to the core functionality of these fluids. Similarly, density changes from heating, or color changes when electrified, do not pertain to the unique characteristics of magneto-rheostatic fluids. Therefore, the focus here is squarely on the significant alteration of viscosity under magnetic influence, which is what defines their behavior and utility in technology.