Sunspots are believed to be responsible for which phenomena observed on Earth?

Sunspots are dark spots on the surface of the Sun, caused by magnetic activity that inhibits convection in the solar atmosphere. They are often associated with the Sun's solar cycle, which can influence various phenomena in the solar system, particularly on Earth.

The correct answer, which relates sunspots to the Aurora Borealis and Aurora Australis, is rooted in the fact that these spectacular light displays in the Earth's polar regions are caused by solar wind—streams of charged particles released from the Sun. When there are fewer sunspots, the solar output is more stable; however, periods of increased sunspot activity are often linked to heightened solar wind and subsequently, more intense auroras. This occurs because the magnetic fields associated with sunspots can lead to solar flares and coronal mass ejections, which result in stronger interactions with the Earth's magnetic field, producing the colorful displays of the auroras.

In contrast, the other options do not have a direct connection to sunspot activity. The Van Allen radiation belts are influenced by cosmic rays and the Earth's magnetic field but not specifically by sunspots. Solar eclipses are events caused by the alignment of the Earth, Moon, and Sun rather than sunspot activity. The Earth's magnetosphere is shaped by the solar wind and