Answer
a. the tilt of Earth's axis toward or away from the sun.
Work Step by Step
As the Earth travels around the sun, its axis is always tilted in the same direction. For the northern hemisphere, near mid-June, Earth is tilted most toward the sun; this causes the northern hemisphere to receive more sunlight and, thus, enter summer. Toward mid-December, Earth's northern hemisphere is tilted away from the sun, and this hemisphere then experiences winter. The opposite season occurs in the southern hemisphere, however. For example, if the northern hemisphere is experiencing summer, then the southern hemisphere would be in the midst of winter.
