The correct answer is: in.
Muslims believe in one God named Allah. This means that they believe that Allah is the only God and that there is no other God. They also believe that Allah is the creator of the universe and everything in it.
The other options are incorrect because they do not accurately reflect the Muslim belief in Allah. For example, the option “of” suggests that Muslims believe that Allah is a part of something else, which is not the case. The option “for” suggests that Muslims believe that Allah exists for a reason, which is also not the case. The option “by” suggests that Muslims believe that Allah was created by someone else, which is also not the case.
In conclusion, the correct answer is “in.” This means that Muslims believe that Allah is the only God and that there is no other God.