Concept:The year 621 C.E. is linked to a key event in early Islamic history that changed worship for Muslims.
Explanation:In 621 C.E., the Prophet Muhammad experienced the Isra and Mi'raj (the Night Journey and Ascension).
During this journey, the five daily prayers, known as salat, were ordained by Allah.
This made salat a formal and fixed mode of worshipping Allah for all Muslims.
Therefore, this year is remembered primarily because the obligation of salat was established.
Answer:D. salat became a mode of worshipping Allah.