Final answer:
The 22-year period required for the repetition of sunspot numbers and magnetic polarities is known as the solar cycle.
Step-by-step explanation:
The 22-year period that is needed for both the average number of spots and the sun's magnetic polarity to repeat themselves, where the sun's polarity reverses on each new 11-year sunspot cycle, is known as the solar cycle. This is because the sun undergoes a periodical increase and decrease in sunspot numbers, with magnetic polarities reversing in each subsequent 11-year sunspot cycle. After one full cycle, the sunspots' magnetic fields in both hemispheres reverse, and it takes another cycle for them to return to their original state—hence, a complete cycle spans 22 years. The 22-year period that involves the reversal of the sun's magnetic polarity and the repetition of the average number of sunspots is known as a solar cycle. This period encompasses two approximately 11-year sunspot cycles, during which the number of sunspots on the Sun's surface increases and then decreases. The sun's magnetic polarity reverses with each new 11-year sunspot cycle. Therefore, option (c) "Solar cycle" is the correct answer.