Read the passage and answer the following question:
According to the controversial sunspot theory, great storms on the surface of the sun hurl streams of solar particles into the atmosphere, causing a shift in the weather on earth.
A typical sunspot consists of a dark central umbra surrounded by a lighter penumbra or light and dark threads extending out from center like the spokes of a wheel. Actually, sunspots are cooler than the rest of the photosphere, which may account for their colour. Typically, the temperature in a sunspot umbra is about 4000k, whereas the temperature in a penumbra registers 5500k and the granules outside the spot are 6000k.
Sunspots have been observed in arrangements of one to more than one hundred spots, but they tend to occur in pairs. There is also a marked tendency for the two spots of a pair to have two opposite polarities. Furthermore, the strength of the magnetic field associated with any given spot's size.
Although there is no theory that completely explains the nature and function of sunspots, several models attempt to relate the phenomenon to magnetic fields along the lines of longitude from the north and the south pole of the sun.
What is the author's main purpose in the passage?

Correct option is A

The author's main purpose in the passage is option (a) to describe the nature of sunspots.

This option is the most accurate as the passage provides detailed descriptions of sunspots, including their physical characteristics such as the umbra and penumbra, their temperatures, and the occurrence of sunspots in pairs with opposite polarities. It also discusses the magnetic fields associated with sunspots and touches briefly on theories relating to their nature.

The focus is on describing various aspects of sunspots rather than proposing new theories, comparing specific parts in detail, or arguing for a particular aspect of sunspots.