Final answer:
Solar activity and the number of sunspots peak approximately every 11 years during the sunspot cycle.
Step-by-step explanation:
The correct answer is a) 11 years. Solar activity and the number of sunspots peak approximately every 11 years. This is known as the sunspot cycle.
During a sunspot maximum, the number of sunspots is typically higher, and during a sunspot minimum, the number is lower. The exact mechanism behind the sunspot cycle is not yet fully understood, but it is connected to the behavior of the Sun's magnetic field.