The best statement that summarizes the causes and effects of the expansion of the Umayyad Caliphate is:
"Because of leadership changes and military conquest, the Umayyad Caliphate expanded to Syria and centralized Islamic power."
This statement is correct because it highlights two key factors: leadership changes, which refer to the transition of power and authority within the Umayyad dynasty, and military conquests, which played a crucial role in expanding the caliphate's territory. The Umayyad Caliphate indeed expanded to Syria, where it established its capital in Damascus, consolidating Islamic power and governance in the region. This expansion was significant in shaping the political and cultural landscape of the early Islamic world.
Further, the other statements either incorrectly mention regions (the Arabian Peninsula or Iraq) or other factors that weren't as pivotal as the leadership and military aspects in fostering the expansion and centralization of authority.