Step 1: Concept
Solar radiation measurement requires specific instruments depending on whether the radiation is direct, diffused, or total (global).
Step 2: Meaning
Global radiation includes both the direct beam from the sun and the diffuse radiation scattered by the atmosphere.
Step 3: Analysis
A pyranometer is designed to measure this total solar irradiance from a $180^{\circ}$ field of view. In contrast, a pyrheliometer measures only direct beam radiation by tracking the sun.
Step 4: Conclusion
Therefore, the pyranometer is the correct instrument for measuring global radiation.
Final Answer: (A)