Radioactivity is measured by

CONCEPT:

• The instrument which is used to measure the radioactivity is called as GM Counter.
• The instrument which is used to measure angle of rotation caused by passing polarized light is called as Polarimeter.
• The instrument which is used to measure the absorbance of particular wavelengths of light is called as Colorimeter.
• The instrument which is used to measure the heat of chemical reactions is called as Calorimeter.

EXPLANATION:

• As the radioactivity is measured by GM Counter. So option 1 is correct.