The most accurate response regarding the growth of Islam and Christianity in Europe between 300 and 1500 CE is:
Both religions were widespread on the continent at different points in time. Islamic empires flourished in Spain under Islamic rule and during the Ottoman Empire. Christian kingdoms flourished across most of Europe apart from these areas and regained a foothold in Spain after the Reconquista.
This response captures the historical context in which both religions were prominent, highlighting the influence of Islamic empires in parts of Europe, especially in Spain and the Balkans, while also acknowledging the dominance of Christianity across much of the continent, particularly after the fall of Islamic rule in parts of Spain during the Reconquista.