Answer: The correct answer for the blank is- A-solar output changes and Milankovitch cycles.
An ice age can be described as a prolonged reduction in the temperature of the atmosphere and Earth's surface, which leads to the expansion of polar ice sheets.
Fluctuations in the amount of incident rays of sunlight ( incoming sunlight) is considered as the most likely reason behind changes in Earth's climate.
In other words, alteration in the duration and the intensity of heat from the sun is the cause of glacial cycles.
The Milankovich cycles occur primarily due to changes in the shape of the Earth's orbit around the sun, which relates the tilted axis of Earth.
Thus, option A) is the right answer.