The question asks which allotrope of carbon is isomorphous, or has the same crystal structure, as crystalline silicon.

Option 1: Coke is a solid carbonaceous material, but it does not have the same crystal structure as silicon. It is not isomorphous with silicon.

Option 2: Diamond is the correct answer. Diamond is a form of carbon that consists of a three-dimensional arrangement of carbon atoms bonded together in a tetrahedral lattice structure. This crystal structure is similar to that of silicon, making diamond isomorphous with crystalline silicon.

Option 3: Graphite is another allotrope of carbon. It consists of layers of carbon atoms arranged in a hexagonal lattice. Although both graphite and silicon have layered structures, they are not isomorphous with each other.

Option 4: Coal is a solid fossil fuel composed mainly of carbon, but it does not have the same crystal structure as silicon. It is not isomorphous with silicon.

Therefore, the correct answer is Option 2: Diamond. It is the only allotrope of carbon that is isomorphous with crystalline silicon.