The answer to this is B. A Compound. A compound is composed of two or more atoms depending on the two elements that are mixed together in order to mix it. These elements are pure which means that a compound is also pure. Take note that the elements that are present in the compound are usually fixed.
For instance, water will not be formed unless there are two hydrogen atoms and one oxygen atom. They all need to bond with each other in order to form H20. There are different ratios that are available for the different type of compounds that are used. If you are trying to make compounds, it is best to know how much is needed to create what you need.