We have that both cubic forms are applied to crystallic structures where all the atoms are the same. They have different packing properties, but no different atoms appear in these models. A substitutional alloy is an alloy where one atom place is filled by another atom that has similar enough properties (physical and chemical) that it does not harm the crystalline structure. An interstitial alloy is an alloy where small atoms are found between the gaps of larger atoms. This is usually for stability and durability reasons; a prime example of this is steel, where the small carbon atoms help iron be more durable. There is a discrepancy in size of atoms in this case of alloy, so the correct answer is D).
