The correct answer is - Caliphs were able to spread Islam as far as Europe and India.
The Caliphs had huge importance int the spreading of the Islamic faith. They managed to spread the new faith with a lightning speed, and they managed to spread it on a very big space. In the period when the Caliphs were ruling, the Islamic faith spread all over the Middle East, penetrated in Europe through Spain, took over North Africa, most of Central Asia, and went as far east as the northwestern part of India.