Final answer:
Both mixing sand and gravel and snowflakes forming in a cloud involve changes in temperature, aligning with option (c). This is due to the transformation that temperature can bring about in the state of matter in the natural world.
Step-by-step explanation:
The two changes mentioned in the question, mixing sand and gravel and snowflakes forming in a cloud, have something in common.
The correct answer is option (c): Both involve changes in temperature. In the case of snowflake formation, a decrease in temperature in the cloud leads to the formation of ice crystals that constitute snowflakes. While mixing sand and gravel does not directly involve a temperature change, it is associated with weathering processes that can be influenced by temperature variations. Although both processes involve nature and changes in states of matter, they are not necessarily related to the other options provided: erosion, the formation of crystals, or the mixing of different substances in the context of the question.