Sort the four by band gap — that is the whole classification.

The two that are not semiconductors are therefore SiO₂ and copper — option 3.

SiO₂ is the archetypal insulator of the entire industry. A 9 eV gap means essentially no thermally generated carriers at room temperature, and that is exactly what makes it invaluable: it is the gate dielectric of every MOSFET, the field oxide isolating adjacent devices, and the mask against dopant diffusion. Note how easily it is confused with silicon itself, whose gap is 1.12 eV — the oxide's extra oxygen changes everything.

Copper has no gap at all. Its valence and conduction bands overlap, so electrons occupy states immediately above the Fermi level and are free to move under any applied field. Its conductivity of about \(5.8\times10^{7}\) S/m is some twenty orders of magnitude above intrinsic silicon's, and unlike a semiconductor its resistance rises with temperature because increased lattice vibration scatters the already-abundant carriers.

ZnO is a genuine semiconductor, and a useful one — its wide gap makes it transparent to visible light while still conducting when doped, which is why it appears as a transparent electrode in solar cells and displays, and in UV photodetectors and gas sensors.

Carbon nanotubes are the interesting case. A nanotube is a rolled sheet of graphene, and whether it is metallic or semiconducting depends entirely on the chirality — the direction in which the sheet is rolled, described by the indices (n, m). If \(n-m\) is a multiple of 3 the tube is metallic; otherwise it is semiconducting with a gap inversely proportional to the diameter. Roughly a third of randomly grown tubes come out metallic, and separating the two kinds is one of the standing difficulties in nanotube electronics.

Hence, the materials that are not semiconductors are (c) SiO₂ and (d) copper.