Correct Answer:
C. Geiger Muller Counter
Radioactivity is measured by a Geiger Muller Counter (C), often simply called a Geiger counter. This instrument detects and measures ionizing radiation, such as alpha, beta, and gamma particles. A Barometer (A) measures atmospheric pressure. A Hygrometer (B) measures humidity in the air. An Ammeter (D) measures electric current. Therefore, the Geiger Muller Counter is the specific device designed for detecting and quantifying radioactivity, making the other options incorrect.