Since there are no options given, here is a brief overview about the 100,000 year cycles of ice age.
Scientists have been making great effort but still find difficulty in finding the reasons why an ice age happens every 100,000 years. True to the Earth's history, warm period and ice age had been regularly alternated. When the Earth roughly cools every after 100,000 years, it swings back ---it gets warmer and eventually, ice melts.
Scientist believed that the reason why the earth has repeating 100,000-year cycles of ice ages is due to orbit change and tilt of the Earth on its axis.