Match solid-state devices to their typical applications: (A) Zener diode, (B) Tunnel diode, (C) LED, (D) Photovoltaic cell — with (1) Used in seven-segment displays, (2) Used in voltage regulator circuits, (3) Used in solar energy utilization, (4) Used in high-speed/digital circuits.

Introduction:
Many diodes have specialized structures that make them ideal for particular applications. Knowing these associations speeds up component selection and circuit reasoning during design and troubleshooting.

Given Data / Assumptions:

• Zener diodes provide near-constant voltage over a range of currents (reverse breakdown).
• Tunnel diodes exploit negative resistance for very high-speed switching/oscillation.
• LEDs emit visible/IR light, commonly used in segment displays.
• Photovoltaic cells convert light to electrical energy.

Concept / Approach:

Zener → voltage regulation/reference. Tunnel diode → microwave/high-speed/digital circuits due to negative differential resistance. LED → segment/panel indicators. PV cell → solar energy harvesting.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets and application notes align with these standard uses; lab kits and textbooks show Zener regulation, LED display drivers, and PV demos.

Why Other Options Are Wrong:

Swapping these roles contradicts device physics (e.g., LEDs are emitters, not regulators; Zeners do not act as detectors/generators of light).

Common Pitfalls:

Confusing tunnel diodes with Schottky or PIN diodes; mixing photovoltaic (power generation) with photodiodes (detection).

Final Answer:

A-2, B-4, C-1, D-3