The correct answer is option 1 - GM Counter.

A GM Counter, also known as a Geiger-Muller counter, is a device used to measure radioactivity. It consists of a tube filled with gas and an electrode, which generates an electric pulse when radiation interacts with the gas. The number of pulses generated is proportional to the intensity of the radiation, which allows the GM Counter to measure the level of radioactivity.

Option 2, a polarimeter, is used to measure the rotation of polarized light caused by optically active substances. It is not used to measure radioactivity.

Option 3, a calorimeter, is used to measure heat transfer or heat release in a chemical or physical process. It is not used to measure radioactivity.

Option 4, a colorimeter, is used to measure the concentration of a colored substance in a solution by measuring the absorbance of light at a specific wavelength. It is not used to measure radioactivity.

Therefore, the correct device for measuring radioactivity is the GM Counter.