The Earth's magnetic field undergoes reversals, where the north and south magnetic poles switch places. These reversals do not occur at regular intervals, but on average, the Earth's magnetic field reverses approximately every 200,000 to 300,000 years. However, if we are to choose from the options provided, the closest answer would be:
C) About every two million years.
While it's important to note that reversals can happen over various time periods, two million years is a rough estimate that fits within the range of observed data.