Explanation
- A
Photo cell
(a) Photo cell. Fire alarms use a photocell (optical smoke detector: LED/beam and photodiode) or an ionisation chamber. Among the given list the photocell is the working sensor. That is the official key.
- B
Water
(b) Water. Water is a fire-fighting medium, not the detecting element of an alarm.
- C
Solar cell
(c) Solar cell. A solar cell converts sunlight to electricity; it is not the smoke/fire sensor.
- D
None of the above
(d) None of the above. A photocell is on the list and is used, so ‘none’ is false.
Summary. Official key is (a). Fire alarms use a photocell (and/or ionisation) to sense smoke. Water and solar cells are not the detector; ‘none’ is wrong.