1. Recall the basic properties of an LED

A light emitting diode (LED) is a heavily doped p-n junction made from a semiconductor such as GaAs, GaP, etc. It emits light due to recombination of electrons and holes when it is forward biased.

2. Check each statement

• Statement A: "It is a heavily doped p-n junction."

  This is correct.

• Statement B: "It emits light only when it is forward biased."

  This is correct. In forward bias, electrons and holes recombine and emit photons.

• Statement C: "It emits light only when it is reverse biased."

  This is incorrect. In normal operation, LED emits light in forward bias, not reverse bias.

• Statement D: "The energy of the light emitted is equal to or slightly less than the energy gap of the semiconductor used."

  This is correct, since photon energy is approximately equal to the band gap: $E = h\nu \approx E_g$ and may be slightly less due to losses.

3. Identify the incorrect statement

The only statement that is not correct is: $\boxed{\text{C}}$

4. Match with given options

The options say:

• A: A
• B: B
• C: C and D
• D: C

So the correct option is: $\boxed{\text{D}}$

5. Comparison with stored answer

Stored correct answer = D

Our derived answer = D

They match.