Thermionic emission:

Thermionic emission is a phenomenon in which electrons are emitted from a heated surface. It occurs when the thermal energy of the surface is sufficient to overcome the work function of the material, causing electrons to be released.

VF Display:

The VF (Vacuum Fluorescent) display is a type of display technology that uses thermionic emission. It consists of multiple layers of thin films, including a cathode and an anode. When a voltage is applied across the cathode and anode, the cathode emits electrons through thermionic emission. These electrons then collide with the phosphor material on the anode, causing it to emit light.

Explanation:

In the given options, only the VF display involves thermionic emission. Let's briefly discuss the other options to understand why thermionic emission does not occur in them.

LEDb:
LED stands for Light Emitting Diode. LEDs are semiconductor devices that emit light when current passes through them. They do not rely on thermionic emission for their operation. Instead, they utilize the phenomenon of electroluminescence, where the recombination of charge carriers (electrons and holes) in the semiconductor material leads to the emission of light.

LCDc:
LCD stands for Liquid Crystal Display. LCDs use liquid crystal molecules to control the passage of light through them. They do not involve thermionic emission. LCDs work by manipulating the alignment of liquid crystal molecules through an electric field, which changes the polarization of light passing through them.

None:
This option indicates that none of the given options involve thermionic emission. It is incorrect because as explained earlier, the VF display does involve thermionic emission.

In conclusion, thermionic emission occurs in VF displays, where electrons are emitted from a heated cathode to produce light when they collide with the phosphor material on the anode. The other options, such as LEDs and LCDs, do not involve thermionic emission for their operation.