Explanation: Tunnel diodes are a special type of diode known for exhibiting a region of **negative resistance** in their forward current-voltage characteristic. In this region, an increase in voltage across the diode causes a *decrease* in the current flowing through it. This unique property of negative resistance allows tunnel diodes to be used for both amplification and oscillation. When properly biased within their negative resistance region and incorporated into an appropriate resonant circuit, they can generate sustained oscillations by counteracting circuit losses. This negative resistance also provides the gain necessary for amplification. Zener diodes (A) are primarily used for voltage regulation by exploiting their reverse breakdown voltage. Point contact diodes (B) were early devices used mainly for detection and rectification. Junction diodes (D) is a broad category, but typical junction diodes are used for rectification, switching, and detection, not for amplification or oscillation via negative resistance.