Radioactivity is measured by

Radioactivity is the spontaneous emission of particles or rays from unstable atomic nuclei. The Geiger–M0ller counter (GM counter) is the primary electronic instrument used for detecting and measuring this ionizing radiation [1]. It operates using a specialized tube that detects nuclear particles in real time. In contrast, other instruments listed serve different scientific purposes: a polarimeter measures the angle of optical rotation in substances using polarized light [2]; a calorimeter is used to measure heat or energy changes in physical or chemical processes; and a colorimeter measures the absorbance of specific wavelengths of light to determine the concentration of a solution. Therefore, the GM counter is the only device among the options designed for radioactivity measurement [4].

Sources

1. [1] https://en.wikipedia.org/wiki/Geiger_counter
2. [2] https://www.sciencedirect.com/topics/physics-and-astronomy/polarimeter
3. [4] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > Radioactivity: > p. 82