Final answer:
Earth's orbital variations, as described by Milankovitch, primarily affect the timing of glacial-interglacial cycles by altering the tilt of Earth's axis.
Step-by-step explanation:
The correct answer is b) Alter the tilt of Earth's axis. Earth's orbital variations, as described by Milankovitch, affect the timing of glacial-interglacial cycles primarily by altering the tilt of Earth's axis. These cycles, known as Milankovitch cycles, can cause changes in the amount of solar radiation the planet receives, leading to changes in climate and the timing of ice ages and interglacial periods.