Which of the following factors affects secular variation?

Secular variation refers to the long-term, gradual changes in the Earth's magnetic field over time, and it is significantly influenced by geomagnetic activity. This variation can be observed over periods of years to centuries and is primarily the result of processes occurring within the Earth's outer core, which generates the geomagnetic field.

Geomagnetic activity is linked to changes in the motion of molten iron and other materials within the outer core due to convection and the Earth's rotation. These internal dynamics lead to variations in magnetic intensity and direction, reflecting how the geomagnetic field evolves over time. Consequently, geomagnetic activity is the key factor affecting secular variation, as it directly relates to the sources of the magnetic field and its fluctuations.

In contrast, atmospheric pressure, Earth's axial tilt, and solar radiation have limited or indirect impacts on the Earth's magnetic field. While solar radiation can influence the magnetosphere temporarily (e.g., during solar storms), it does not cause the long-term changes that characterize secular variation. Similarly, atmospheric pressure and axial tilt relate more to meteorological and astronomical phenomena rather than the geomagnetic processes responsible for secular variation.