Concept:A Geiger-Müller (GM) counter detects and measures ionizing radiation from radioactive substances.Explanation:The GM counter works by registering electrical pulses produced in a gas-filled tube when radiation passes through it.Other instruments have different uses: a polarimeter measures optical rotation, a calorimeter measures heat of chemical reactions, and a colorimeter measures light absorbance.Thus, only the GM counter is directly used for measuring radioactivity.Answer:Option A: GM Counter.