The best response would be:
Both religions were widespread on the continent at different points in time. Christian empires flourished in Spain under Islamic rule and during the Ottoman Empire. Islamic kingdoms flourished across most of Europe apart from these areas and regained a foothold in Spain after the Reconquista.
This response accurately highlights the complexities of the growth of both Islam and Christianity in Europe between 300 and 1500 CE, acknowledging the coexistence and influence of both religions during different periods, particularly in regions like Spain where Islamic rule was significant. It also recognizes the role of the Reconquista in shaping the religious landscape of Europe.