Discussion
Why must a light emitting diode be driven through a series resistor?
Answer: D
The forward characteristic is so steep that a small voltage change produces a large current change, so the current must be set externally rather than by the applied voltage. The low reverse rating matters for protection, the temperature dependence affects brightness, and the colour-dependent drop matters when choosing the resistor's value - but none of them is why the resistor is needed. Any device with a steep characteristic must have its operating point set by something in series.
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