A compound is best described as: c) a material that is made up of a combination of atoms bonded together.
Explanation: In chemistry, a compound is formed when two or more different types of atoms bond together in fixed proportions. This bonding creates a new substance with its own unique properties, distinct from the individual elements that compose it. For example, water (H2O) is a compound made up of hydrogen and oxygen atoms bonded together. Each of the other options provided describes different concepts in chemistry, such as mixtures or elements, which do not accurately define a compound.