Concept:A p-n junction diode is used for rectification, photodetection, and light emission, but not for amplification.
Explanation:A p-n junction diode works as a rectifier by converting a.c. to d.c., so option A is possible.
A photodiode or solar cell, based on a p-n junction, converts light energy into electrical energy, so option B is possible.
An LED, also a p-n junction device, emits light radiation, so option C is possible.
Amplifying the amplitude of an a.c. signal requires a transistor or amplifier circuit, not a simple p-n junction diode.
Therefore, a p-n junction diode cannot be used for increasing the amplitude of an a.c. signal.
Answer:Option D is correct.