‘Pyrheliometer’s is used for measuring:

The correct answer is B) Solar radiation. A pyrheliometer measures direct beam solar irradiance, the radiation arriving from the sun's disc. It has a collimating tube with a narrow field of view (about 5 degrees), so it sees only the sun and a small surrounding annulus, and a thermopile or cavity detector records the power per unit area. Because it must face the sun continuously, it is mounted on a solar tracker.

Why the others are wrong:

• A) Sun spots are dark, cooler patches on the photosphere. They are observed with telescopes and tracked through sunspot number records, not measured with a pyrheliometer.
• C) Air temperature is measured with a thermometer kept in a Stevenson screen, about 1.25 to 2 m above ground.
• D) Temperature of plants is measured with leaf thermocouples or infrared thermometers, not radiometric instruments of this kind.

Core concept: radiometry, the measurement of radiant energy. A pyranometer measures global radiation on a horizontal surface, while a pyrheliometer measures the direct component. For Mains, this links to solar energy policy, the National Solar Mission, and radiation data for agriculture and climate studies.

Trap: candidates pick "Sun spots" because the name sounds astronomical. The "helio" in pyrheliometer only means sun; the instrument measures radiation.