The correct answer is (b). A pyrheliometer is a device used to measure the direct, normal incidence of solar radiation. It is a type of pyranometer, which is a device that measures solar radiation. Pyrheliometers are used in a variety of applications, including meteorology, astronomy, and agriculture.
(a) Sun spots are dark areas on the surface of the sun. They are caused by magnetic activity on the sun. Sun spots can be observed with a telescope or with a special camera called a solar filter.
(c) Air temperature is the temperature of the air. It is measured with a thermometer. Air temperature can vary depending on the time of day, the season, and the location.
(d) The temperature of plants is the temperature of the plant’s tissues. It is measured with a thermometer. The temperature of plants can vary depending on the type of plant, the time of day, and the environment.