"do not argue with the followers of earlier revelation otherwise than in a most kindly manner-unless it be such of them as are bent on evil-doing-and say: 'We believe in that which has been bestowed from on high upon us, as well as that which has been bestowed upon you; for our God and your God is one and the same.'" --Qur'an 29:46

Read the passage from the Qur'an. The "followers of earlier revelation" are Jews and Christians, illustrating that Muslims believe that
a.)
there is more than one true God.
b.)
Jews and Christians worship idols.
c.)
Jews, Christians, and Muslims worship the same God.
d.)
Jews and Christians worship a different God than Allah.