A. Both religions believe in only one god.
This option is the most accurate in terms of similarity between Islam and Christianity, as both religions are monotheistic and believe in the existence of one God. Option B is partially correct, as both religions practice fasting, but the nature and practices of fasting differ. Option C is incorrect because, in Christianity, Jesus is considered the Son of God and part of the Holy Trinity, while in Islam, Jesus (Isa) is regarded as a prophet, not the only prophet. Option D is also incorrect, as Islam views Mecca as the holy city, while Christianity does not have a similar designation for Mecca.