The correct answer is Diamond.Concept:Allotropy refers to the ability of chemical elements to exist in two or more forms with the same physical state.Explanation:Diamond and crystalline silicon are isomorphous.Crystals are described as isomorphous in crystallography if their shapes are very similar. Carbon and silicon are both members of the same group.The C atom in diamond and the Si atom in crystalline silicon is coordinated with four other atoms, resulting in a tetrahedral structure that extends over a wide range.Because they both have three-dimensional tetrahedral structures, diamond and crystalline silicon are isomorphous.Diamond has a very distinct and well-defined shape, as well as a well-defined structure. The structure of a diamond is shown below: Additional InformationWhile the other two, coke and coal structure is not well defined because they are carbon combustion products obtained through different processes, whereas graphite has a layered structure.Coal is formed naturally through the decay of plants, animals, and fossils, whereas coke is a byproduct of coal combustion.