B. Islam was founded on the teachings of Muhammad.
C. The holiest city of Islam is Mecca.
Muhammad is regarded as the founder of Islam since he was the one who received God's messages in the forms of divine revelation given by the Angel Gabriel in the Mecca and Medina cities, starting at the age of 40 years old. Muhammad wrote all of these revelations down and they are part now of the Holy Qur'an, a book whereby Islam is based so it is fair to say that Islam was founded on his teachings.
And Mecca is considered the holiest city in the religion of Islam because it was the birthplace of Muhammad and the place where he stated that Allah revealed their holy book.
