How is a compound defined in terms of its composition?

A compound is defined as a substance that consists of two or more different elements that are chemically combined in a fixed ratio. This chemical bonding means that the elements lose their individual properties and create a new substance with unique characteristics. For example, water (H₂O) is a compound formed from hydrogen and oxygen atoms bonded together in a specific arrangement, different from the properties of the two gases alone.

The correct answer emphasizes the essential aspect of compounds: the chemical combination of distinct elements. This distinguishes compounds from other forms of matter, such as mixtures, where the components retain their individual properties and can vary in proportion. In a mixture, the elements are not chemically bonded, contrasting sharply with the definition of a compound.

The other options all address different types of matter or describe characteristics that do not align with the definition of a compound. A compound must involve elements that are chemically joined together, which is not the case for a single element, a mineral with a crystalline structure, or a homogeneous mixture of minerals. The specific nature of a compound's composition highlights its significance in the study of chemistry and materials.