The best response summarizing the fundamental beliefs of Islam and its religious practices is:
"The Quran is a sacred scripture and teachings that all Muslims follow. The primary-source scripture is the first pillar: profession of faith."
This response highlights the Quran's role in Islamic belief and its connection to the first pillar of Islam, which is the declaration of faith (shahada) stating "There is no god but Allah, and Muhammad is the Messenger of God."