The best response that accurately reflects 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 statement covers the major historical developments of both religions during this period, including the spread of Islam in the Iberian Peninsula and the influence of the Ottoman Empire, while also recognizing the dominance of Christianity in most of Europe during this timeframe.