Which of the following photoelectric devices is most suitable for digital applications?

Correct answer: (a) Photodiode

Explanation

A photodiode is a light detector: its current switches sharply on and off as light falls on it or is cut off, and it responds very fast. So it suits digital (switching and control) uses, such as optical receivers and counters.

A photovoltaic (solar) cell is made to give electrical power from sunlight, not to switch signals, and a photoemitter (such as an LED) turns electrical energy into light.

So option (a), the photodiode, is the most suitable.