Which of the following is used in fire alarm?

The correct answer is A) Photo cell. A photoelectric smoke detector works on a simple principle. A light beam or LED sits inside a sensing chamber, and a photocell (light-sensitive sensor) is placed so it normally receives little light. When smoke enters, it scatters the light onto the photocell, the change in output is detected, and the alarm sounds.

• B) Water is wrong. Water is the extinguishing agent in sprinklers and hydrant systems. It is not the detecting element of an alarm.
• C) Solar cell is wrong. A solar cell converts light into electricity. It may power a standby supply for an alarm, but it does not sense fire.
• D) None of the above is wrong because option A is correct.

The core concept is the difference between detection and suppression. Other detectors use ionisation chambers with a radioactive source, or heat sensors such as thermistors or bimetallic strips. The National Building Code of India 2016 sets fire detection and alarm requirements for buildings, which links this to urban safety and disaster management questions in Mains.

Trap: Aspirants often pick water because it is so closely linked with fire fighting. Keep detection and suppression separate in your mind.