Final answer:
A mineral's crystal structure means that its atoms are arranged in an orderly, repeating pattern, giving the mineral its characteristic geometric shapes. This crystal structure is a fundamental trait alongside having a defined chemical formula and being an inorganic solid.
Step-by-step explanation:
One of the characteristics that define a mineral is that it has to have a crystal structure. This means the atoms are arranged in an orderly, repeating pattern, which is aspect (c) of your options. Unlike substances that are liquid or vapor, minerals are inorganic solids with a specific internal structure that is visible externally as geometric shapes with defined faces and angles. These crystal structures result from the way the atoms or molecules are packed together. A defined chemical formula, characteristic for each mineral, also reflects this orderliness.
For example, halite always has the chemical formula NaCl (sodium chloride), representing its composition of sodium and chlorine atoms in a particular ratio and arrangement. Some minerals, like olivine, can have a range of compositions due to the substitution of similar-sized and charged atoms, but still have a defined crystal structure. Therefore, while a mineral may be composed of multiple elements (option a) and has a defined chemical formula (option b), it is the repeating pattern of atoms that is key to its crystal structure (option c), and while it must be naturally occurring (option d), this is not the defining feature related to crystal structure.