Tunnel diode shows the negative characteristics in forward bias. It is used in forward bias. Avalanche photo diode is used in reverse bias.

Tunnel diode and avalanche photo diode are operated in X bias and Y bias, respectively.

To understand the correct substitutes for X and Y, we need to understand the operating characteristics of tunnel diodes and avalanche photodiodes.

1. Tunnel Diode:
A tunnel diode is a heavily doped p-n junction diode that exhibits negative resistance. It is operated in a region called the negative resistance region, where the current decreases with an increase in voltage. The tunnel diode can be biased in two ways:

- Reverse Bias: In reverse bias, the positive terminal of the voltage source is connected to the n-side of the diode, and the negative terminal is connected to the p-side. This biasing condition allows the tunneling of electrons through the potential barrier, resulting in a large current flow.

- Forward Bias: In forward bias, the positive terminal of the voltage source is connected to the p-side of the diode, and the negative terminal is connected to the n-side. This biasing condition blocks the tunneling of electrons through the potential barrier, resulting in a very small current flow.

2. Avalanche Photodiode:
An avalanche photodiode is a photodiode that operates in the avalanche breakdown region. It is designed to provide internal amplification of the photocurrent. The avalanche photodiode can also be biased in two ways:

- Reverse Bias: In reverse bias, the positive terminal of the voltage source is connected to the p-side of the diode, and the negative terminal is connected to the n-side. This biasing condition allows the diode to operate in the avalanche breakdown region, where the photocurrent is multiplied due to impact ionization.

- Forward Bias: In forward bias, the positive terminal of the voltage source is connected to the n-side of the diode, and the negative terminal is connected to the p-side. This biasing condition prevents the diode from operating in the avalanche breakdown region and results in very low or no amplification of the photocurrent.

Correct substitutes:
Based on the above explanations, the correct substitutes for X and Y are:

- X: forward (for tunnel diode)
- Y: reverse (for avalanche photodiode)

This is because tunnel diodes are operated in forward bias to block tunneling, and avalanche photodiodes are operated in reverse bias to enable avalanche breakdown and amplification of the photocurrent.

Hence, the correct answer is option 'C': X: forward, Y: reverse.