What is the forbidden energy gap (${E_g}$) or band gap?
A. Forbidden energy gap ${E_g}$​ is the difference of energy levels of conduction band and within conduction band
B. Forbidden energy gap ${E_g}$ is the difference of energy levels of conduction band and valence band.
C. when there is no energy
D. conduction band

Hint – In order to solve this problem we need to draw the diagram of conduction band and valence band and then the portion of the energy band structure of a semiconductor or insulator between the valence and conduction bands is forbidden energy gap.

Complete Step-by-Step solution:
The diagram of conduction and valence band can be drawn as:

Forbidden energy gap, also known as band gap refers to the energy difference (eV) between the top of valence band and the bottom of the conduction band in materials. Current flowing through the materials is due to the electron transfer from the valence band to the conduction band.
Hence, the correct option is B.

Note – To solve such problems we just need to know that Forbidden energy gap, also known as band gap refers to the energy difference (eV) between the top of valence band and the bottom of the conduction band in materials. Current flowing through the materials is due to the electron transfer from the valence band to the conduction band.