C. If it doesn't easily combine with other elements is the main reason why a metal can exist in a free state. If a metal has low reactivity, it will not easily combine with other elements, and therefore can exist in a pure form. However, options A, B, and D can also play a role in a metal existing in a free state. Some metals have a high enough mass to be found in nature on their own, while other metals are highly reactive and may be found in a free state as a result. Additionally, some metals are commonly used to make alloys, and can still be found in a free state.