The best explanation for why the Southern Hemisphere experiences seasons opposite to those in the Northern Hemisphere is:
The tilt of Earth's axis is reversed in the Southern Hemisphere.
This statement is a bit misleading, as it doesn't mean the tilt itself is reversed; rather, the same tilt causes different hemispheres to receive varying amounts of sunlight during different times of the year. When the Northern Hemisphere is tilted toward the Sun and experiences summer, the Southern Hemisphere is tilted away and experiences winter, and vice versa.
The other options provided do not accurately explain the seasonal differences between the hemispheres.