Which of the following materials can flow downslope?

The correct choice is C, which includes mud, soil, volcanic ash, and debris because all of these materials can flow downslope under certain conditions due to the force of gravity. This group of materials is often involved in mass wasting processes, which are movements of earth materials down slopes. For instance, when saturated with water, mud and soil can lose cohesion and begin to flow, creating mudflows. Volcanic ash can also behave like a fluid when disturbed, especially if it is mixed with water. Debris flows, which consist of a mixture of water, rock, and organic matter, are another example of how various terrestrial materials can mobilize and flow together.

Other options do not encompass the broader categories of materials that can flow downslope. While solid rock can sometimes move as part of larger landslides, it is not typically described as flowing in the same way as liquids or granular materials. Ice and snow can flow in specific contexts, such as glaciers, but they are not classified as typical flowing earth materials in a geotechnical context. Water, while it does flow, represents only one type of moving material and does not account for the variety of other substances that can also move downslope under the right conditions.