Through a semiconductor, an electric current is due to the drift of
A. free electrons
B. Positive and negative ions
C. free electrons and holes
D. protons

Hint: In semiconductors there is an energy gap between the valence band and the conduction band. As the electrons in valence band obtain enough energy to go from the valence band to the conduction band current starts flowing due to drift of electrons from valence band to the conduction band and of holes from the conduction band to the valence band.

Complete step by step solution:
Electron current and hole current, respectively, produce current in semiconductors. When electrons are driven from the negative terminal to the semiconductor, an electron current is created. In a semiconductor, holes are atom vacancies with the ability to receive electrons.

The electrons in both the electron current and the hole current move from the negative terminal to the positive terminal. However, holes are thought of as hypothetical particles with a positive charge that appear to move in the opposite direction of electrons, from the positive to the negative terminal.

Thus, free electrons and holes are the charge carriers in semiconductors. The diffusion of charge carriers (holes and/or electrons) in a semiconductor results in a current known as diffusion current. This current results from the transfer of charges, that is, transfer of electrons from the valence band to the conduction band or that of holes from the conduction band to the valence band, that happens as a result of the semiconductor's non-uniformly distributed charge.

Hence, the correct answer is option C.

Note: One should not confuse ions as free electrons and holes. A band gap exists in semiconductors between the two bands which is an energy range where energy quantization prevents the existence of any electron states in the material.