The historical factor that significantly contributed to the establishment of the modern state of Israel is:
b. the attempted genocide of European Jews in the Holocaust.
The Holocaust during World War II resulted in the systematic murder of six million Jews and had a profound impact on global Jewish consciousness, leading to increased support for the establishment of a Jewish homeland in Palestine. This support was instrumental in the eventual declaration of the state of Israel in 1948.
While other historical events mentioned (like the Crusades or the Reconquista) had their own influences on Jewish history, their direct relation to the establishment of modern Israel is not as significant as the impact of the Holocaust.