Step-by-step explanation:
a. Cloud chamber:
"A cloud chamber, also known as a Wilson cloud chamber, is a particle detector used for visualizing the passage of ionizing radiation".
b. Film badge:
Is a personal dosimeter used for monitoring cumulative radiation dose due to ionizing radiation.
c. Nuclear reactor:
Is a device used to perform fission or fusion nuclear reactions.
d. Geiger counter:
"A Geiger counter is an electronic instrument used for detecting and measuring ionizing radiation."
All of them are used to detect radioactivity except from the nuclear reactor that is used to perform a nuclear reaction.
Answer: c. nuclear reactor