Which of these BEST explains a reason why the Sunnah is an
important book for Muslims?

Sunnah, (Arabic: “habitual practice”)also spelled Sunna, the body of traditional social and legal custom and practice of the Islamic community. Along with the Qurʾān (the holy book of Islam) and Hadith (recorded sayings of the Prophet Muhammad), it is a major source of Sharīʿah, or Islamic law.

In pre-Islamic Arabia, the term sunnah referred to precedents established by tribal ancestors, accepted as normative and practiced by the entire community. The early Muslims did not immediately concur on what constituted their Sunnah. Some looked to the people of Medina for an example, and others followed the behaviour of the companions of the Prophet Muhammad, whereas the provincial legal schools, current in Iraq, Syria, and the Hejaz (in Arabia) in the 8th century CE, attempted to equate Sunnah with an ideal system—based partly on what was traditional in their respective areas and partly on precedents that they themselves had developed. These varying sources, which created differing community practices, were finally reconciled late in the 8th century by the legal scholar Abū ʿAbd Allāh al-Shāfiʿī (767–820), who accorded the Sunnah of the Prophet Muhammad—as preserved in eyewitness records of his words, actions, and approbations (the Hadith)—normative and legal status second only to that of the Qurʾān.

The authoritativeness of the Sunnah was further strengthened when Muslim scholars, in response to the wholesale fabrication of hadiths by supporters of various doctrinal, legal, and political positions, developed ʿilm al-ḥadīth, the science of determining the reliability of individual traditions. The Sunnah was then used in tafsīr (Qurʾānic exegesis) to supplement the meaning of the text and in fiqh (Islamic jurisprudence) as the basis of legal rulings not discussed in the Qurʾān.