The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
What caused Amundsen to change his mind to sail to the South Pole rather than the North Pole was the fact that he knew that other explorers had already arrived at the North Pole. They were Robert E. Peary and Frederick Cook. So in 1909, his plan had to be changed and he decided to explore the South Pole. But to keep the support and fundraising of his people, he decided to keep this expedition in secret, not really knowing how long this expedition would take.
Roald Amundsen finally arrived at the South Pole on December 14, 1911.