Based on the statements provided regarding the spread of Islam, the most accurate option would be:
B. After the death of the prophet Muhammad, Islam spread as far north as present-day Portugal and Spain.
This statement aligns with historical events, as Islam expanded rapidly after the death of Muhammad in 632 A.D., reaching parts of Europe, including the Iberian Peninsula (modern-day Spain and Portugal), within a few decades. The other options contain inaccuracies about the timeline and impact of Islam's spread.