Answer: D. Cubic boron nitride (c-BN) has the same structure as diamond — a giant covalent network where each atom is bonded to four others by strong covalent bonds. Diamond's hallmark properties (extreme hardness, very high melting point, electrical insulator) carry over to c-BN.
Use 2 (abrasive) follows directly from extreme hardness. Use 3 (cutting hard steels) also follows — only something at least as hard as the workpiece can cut it.
Use 1 (electrical conductor) is wrong. All electrons in c-BN are localised in covalent bonds, just as in diamond. There are no free electrons or mobile ions, so c-BN is an electrical insulator.
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