Which compound has the highest melting point?
silicon carbide (sic), a network solid
octane (c8h18), a molecular compound
carbon dioxide (co2), a common gas
water (h2o)

silicon carbide (SiC), a network solid

Explanation:

SiC has a giant network of covalent structure which takes a lot of energy to break than just a single covalent bond. In the network there are many Si and C atoms. All the atoms in the network are linked to each other by strong covalent bonds. SiC forms a lattice structure, in which both the Si and C atoms are sp³ hybridized. In the SiC crystal lattice the smallest unit is a tetrahedron of a central Si atom surrounded by four C atoms or a central C atom surrounded by four Si atoms.

answer is: naturally occurring solid, inorganic compound or element is mineral.

minerals usually have crystalline form, that means than crystal constituents (atoms, molecules or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions.

for example magnetite is a mineral of iron oxide (fe₂o₃).

calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (caco₃).

dolomite is an   carbonate mineral composed of calcium magnesium carbonate (camg(co₃)₂).