What was the most significant cause of the Ilkhanate's collapse?

The most significant cause of the Ilkhanate's collapse was **d. The Ilkhanate collapsed because it was devastated by the bubonic plague**. The plague severely weakened the population and economy of the Ilkhanate, contributing to social unrest and political fragmentation. Additionally, the lack of strong leadership following the death of the ruler Abu Sa'id, who did not have a male heir, further destabilized the region. While several factors contributed to its decline, the impact of the bubonic plague was particularly devastating.