The correct answer is false.
It is false that the caliphs declared conquered people had to convert to Islam.
Indeed, under Islam teachings, conversion cannot be forced to anyone. Islam prohibits to impose conversion. The Caliphates respect other people's religions but invited them to convert through missions. Many people accepted to convert to the Muslim religion in the times when conquer and trade spread Islam to North Africa, Asia, and parts of Europe such as the Iberian peninsula.